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Bertram   C.  8L   SEtntile, 

3LIU3,,  JB.Sc.,  ».*.® 

WHEN  Father  Wasmahn's  work  Die  Moderne  Biologic 
und  die  Entwicklungstheorie  appeared  in  1906  we  wel- 
comed it  in  this  REVIEW  and  expressed  the  hope  that  "so 
important  and  useful  a  book  [might]  shortly  be  translated 
into  English  so  as  to  be  made  available  for  those  who  do. 
not  read  German."  It  is  now  1910,  and  at  last  the  desired 
translation  has  appeared  (Modern  Biology  and  the  Theory  of 
Evolution^  Translated  from  the  Third  German  Edition  by 
A.  M.  Buchanan,  M.A.  London:  Kegan  Paul,  Trench, 
Trubner and  Co,  1910.  Price  i6s.).  As  we  dealt  fully  with 
the  book  in  its  German  dress  it  is  not  necessary  to  repeat 
what  has  already  appeared  concerning  it  in  these  columns* 
Suffice  it  to  say  that  the  translation  appears  to  have  been 
adequately  carried  out  and  that  the  printer  and  publishers 
have  done  their  best  to  present  the  reading  public  with  a 
handsome  volume.  Let  us  be  permitted  to  impress  upon 
those  unacquainted  with  modern  Biological  work  that  in 
the  author  of  this  book  they  have  to  deal  not  merely  with 
the  ordinary  type  of  Christian  apologist,  but  with  a  man 
who — as  his  most  embittered  opponents  are  constrained 
to  admit- — stands  in  the  very  front  rank  of  biological 
Workers,  indeed,  in  his  own  particular  line  has  no  rivaL 
Hence  the  only  method  of  reply  is  that  adopted  by  some 
of  his  critics  who  suggest,  or  even  openly  state,  that  Was- 
mann  the  Scientist  suffers  eclipse  by  Wasmann  the  Jesuit 
where  questions  which  may  be  supposed  to  affect  religion 
are  under  consideration*  In  a  word  the  assertion  is  that 
religious  bias  outweighs  scientific  accuracy*  This  curious 
attitude  is  familiar  enough  in  England,  where  the  present 
reviewer  has  not  seldom  heard  books  that  at  all  diverged 
from  the  fashionable  scientific  opinions  of  the  moment 
spoken  of  in  slighting  tones  as  "books  written  with  & 
bias  against  Darwinism  "'  or  whatever  other  "ism  *'  may 
have  been  in  question.  He  once  asked  a  person  who  had 
made  use  of  this  criticism  whether  he  had  ever  read  a  book 
on  Darwinism  which  was  written  with  a  strong  bias  in 
favour  of  that  system.  The  interlocutor  was  a  man  of  scru- 
pulous honesty,  and,  after  reflecting  for  a  moment,  he  re- 
plied, "Do  you  know  that  never  occurred  to  me  before?" 
In  the  same  manner,  Wasmann  most  properly  replies 
to  his  opponents: 

I  must  acknowledge  that  with  regard  to  the  doctrine  of  creation, 
the  hypothesis  of  spontaneous  generation  and  the  application  of  the 
theory  of  des.cent,  I  had  a  bias,  and  one  that  is  directly  opposed  to 
that  of  my  reviewer.  I  had  the  intention  of  proving  that  a  reason- 
jible  theory  of  evolution  necessitates  our  assuming  the  existence 
of  a  personal  Creator,  and  I  wished  further  to  show  that  "  spon- 
taneous generation  "  was  scientifically  untenable,  and,  therefore, 
could  not  be  a  postulate  of  science.  Finally,  I  desired  to  prove  that 
to  regard  man  from  the  purely  zoological  point  of  view  is  a  one- 

sided  and  mistaken  proceeding.  1  was,  however,  forced  to  adopt 
this  threefold  bias  by  the  monists,  who  were  exerting  themselves 
with  a  much  greater  bias  to  establish  false  philosophical  postulates 
in  the  name  of  biology,  and  to  force  them  as  "  monistic  dogmas  " 
upon  all  interested  in  science.  I  considered  it  my  duty  as  a  Chris- 
tian and  as  a  scientific  man  to  protest  vigorously  against  these  at- 
tempts at  a  fresh  subjugation  of  the  human  intellect "  (p.  xxii). 

It  is  much  to  be  hoped  that  this  book  will  have  a  large 
Sale  in  this  country,  and  certainly  every  Catholic  school 
and  college  which  does  not  include  it  in  its  Library  incurs 
a  grave  responsibility.  As  our  previous  wishes  with  regard 
to  an  English  edition  have  borne  fruit  we  will  venture  a 
further  aspiration,  which  is  that  before  long  this  valuable 
jvork  may  be  available  in  a  much  cheaper  form — -might  we 
dare  even  .to  hope  in  the  popular  sixpenny  form? 



Nihil  Obstat 

Sti.  Ludovici,  die  17  Aprilis,  1910 

P,    G.    ROLWERK 



Sti.  Ludovici,  die  17  Aprilis,  1910 

Archiepus  Sti.  Ludovici 







A.  M.  BUCHANAN,  M.A. 




Nulla  unquatn  inter  fidem  et  ratio- 
nem  vera  dissensio  esse  potest. 

There  can  never  be  any  real  con- 
tradiction between  faith  and  reason. 

(Constitutiones  Concilii  Vaticani,  c.4, 
De  fide  et  ratione.) 

Cum  opus,  cui  titulus  est :  '  Biologie  und  Entwicklungstheorie,' 
editio  tertia,  ab  Erico  Wasmann,  Sacerdote  Soc.  Jesu,  compositum  aliqui 
eiusdem  Societatis  revisores,  quibus  id  commissum  fuit,  recognoverint 
et  in  lucem  edi  posse  probaverint,  facultatem  concedimus,  ut  typis 
mandetur,  si  ita  iis,  ad  quos  pertinet,  videbitur. 

In  quorum  fidem  has  literas  manu  nostra  subscriptas  et  sigillo 
muneris  nostri  munitas  dedimus. 

Exaten,  die  29  mensis  Julii,  1906. 

Prov.  Germ.  Prsepositus. 

( The  rights  of  translation  and  of  reproduction  are  reserved) 


AT  the  present  day  it  is  incumbent  upon  every  educated  man 
to  familiarise  himself  to  some  extent  with  the  progress  made 
and  the  results  attained  by  modern  science,  and  especially  by 
biology.  Only  in  this  way  will  he  be  in  a  position  to  form 
any  opinion  regarding  the  intellectual  contest  that  rages 
round  certain  important  philosophical  problems  arising  out 
of  biology,  namely,  the  comparative  psychology  of  man  and 
beasts  and  the  theory  of  evolution.  I  have  already  dealt 
with  the  former  of  these  two  problems  in  two  special  works, 
intended  for  general  reading,  viz.  :  *  Instinkt  und  Intelligenz 
im  Tierreich  '  ('  Instinct  and  Intelligence  in  the  Animal  King- 
dom ')  (third  edition,  Freiburg  im  Breisgau,  1905),  and 
'  Vergleichende  Studien  liber  das  Seelenleben  der  Ameisen  und 
der  hoheren  Tiere  '  ('  Comparative  Studies  regarding  the  in- 
telligence of  ants  and  the  higher  animals ')  (second  edition, 
Freiburg  im  Breisgau,  1900).  My  aim  in  the  present  work  is 
to  comply  with  wishes  expressed  in  various  quarters,  and  to 
render  my  articles  on  biology  and  evolution  accessible  to 
readers  in  general. 

These  sketches  appeared  originally  as  a  series  of  articles 
in  the  magazine  entitled  Stimmen  aus  Maria-Loach,  1901-3. 
Even  in  their  present  considerably  expanded  form  they  are 
still  sketches,  with  no  pretensions  to  completeness,1  as  they  are 
intended  chiefly  for  readers  who  have  no  special  knowledge 
of  the  departments  of  science  with  which  I  have  dealt.  I  hope, 

1  The  chapter  on  the  relation  between  cellular  division  and  the  problems 
of  fertilisation  and  heredity  has  been  rewritten.  For  much  information 
on  the  subject  of  botany  I  am  deeply  indebted  to  my  colleague,  Father  J. 
Rompel,  S.J.,  Professor  at  the  Stella  Matutina  Gymnasium  at  Feldkirch. 
I  have  received  very  valuable  suggestions  from  other  specialists  in  various 
branches  of  science,  and  I  take  this  opportunity  of  expressing  my  gratitude 
to  them. 


however,  that  these  dissertations  will  be  of  some  use  also  to 
students  attending  lectures  on  biology  and  the  theory  of 
evolution  ;  they  will  find  many  facts  presented  to  them  from 
a  fresh  point  of  view,  and  this  is  particularly  true  of  the  last 
four  sections  on  the  modern  theory  of  evolution.  The  chapter 
headed  '  Theory  of  Permanence  or  Theory  of  Descent  '  is 
based  almost  exclusively  upon  the  results  contained  in  my 
previous  150  special  articles  on  inquilines  or  guests  among  ants 
and  termites,  and  may  be  of  interest  to  my  colleagues  who 
have  made  a  special  study  of  zoology. 

I  trust  that  this  work  will  be  received  in  as  friendly  a 
spirit  as  were  the  two  previously  mentioned  psychological 
works.  In  all  three  alike  I  have  spoken  as  a  Christian  engaged 
in  scientific  research,  and  I  am  firmly  convinced  that  natural 
•truth  can  never  really  contradict  supernatural  revelation, 
because  both  proceed  from  one  and  the  same  source,  viz.  the 
everlasting  wisdom  of  God.  Therefore  the  study  of  modern 
biology  and  of  the  theory  of  descent,  if  carried  on  without 
prejudice,  can  tend  only  to  the  glory  of  God. 



Feast  of  St.  Ignatius,  1904. 


THIS  new  edition  contains  many  corrections  and  additions, 
which  our  increased  knowledge  of  this  branch  of  science  has 
enabled  me  to  make.  The  chapter  on  the  physiology  of 
evolution  and  the  section  on  the  history  of  slavery  amongst 
ants  are  entirely  new.  The  former  throws  some  light  on  the 
problem  of  determination,  and  the  latter  illustrates  the 
application  of  the  theory  of  descent  to  the  development  of 

In  its  present  form  the  book  possesses  more  unity  than  it 
did  before.  The  two  chief  parts,  those,  namely,  on  cytology, 
or  the  study  of  cells,  and  on  the  theory  of  evolution,  are  now 
connected  harmoniously  with  one  another.  The  branch  of 
science  with  which  I  had  to  deal  is,  however,  vast  in  itself, 
and  is  being  enriched  almost  daily  by  the  publication  of  fresh 
works,  so  that  it  is  quite  impossible  to  give  an  exhaustive 
account  of  it  in  a  limited  space.  Similar  considerations  led 
even  E.  B.  Wilson  to  have  the  new  editions  of  his  classical 
work  '  The  Cell '  (1900  and  1902)  reprinted  without  alteration, 
and  so  I  may,  perhaps,  be  forgiven  for  having  made  only  the 
most  absolutely  necessary  corrections  and  additions. 

I  wish  to  emphasise  the  fact  that  it  is  not  my  intention  that 
this  work  should  serve  as  a  complete  textbook  of  the  theory 
of  descent.  The  chapters  on  this  subject  are  intended  only, 
on  the  one  hand,  to  help  the  reader  to  form  a  clear  conception 
of  the  meaning  of  the  theory  of  evolution,  the  philosophical 
and  scientific  principles  underlying  it,  and  its  limits  and 
causes  ;  and,  on  the  other  hand,  to  lay  before  him  fresh  evi- 
dence, derived  from  my  own  special  department  of  biology, 
which  tends  to  prove  that  the  theory  of  evolution  is  really 
better  supported  than  that  of  permanence.  This  theory  of 


evolution,  which  I  regard  as  a  well-founded  hypothesis,  must 
be  polyphyletic  and  not  monophyletic,  if  it  is  to  correspond 
with  known  facts. 

With  regard  to  the  application  of  the  theory  of  descent  to 
man,  I  abide  by  my  previous  opinion,  and  maintain  that  the 
mental  evolution  of  man  from  brutes  is  impossible,  and  that 
his  bodily  descent  from  brute  ancestors  presents,  from  the 
scientific  standpoint,  difficulties  that  have  hitherto  not  been 

In  the  chapter  on  the  Division  of  Cells  new  diagrams  have 
been  substituted  for  those  which  appeared  in  the  earlier 
editions,  and  in  other  places  also  fresh  diagrams  have  been 
added  (fourteen  in  all),  which  are  almost  all  original.  Three 
extra  plates  have  been  added,  viz.  Nos.  II,  VI,  VII. 

Since  the  appearance  of  the  second  edition  it  has  been 
translated  into  Italian  by  Era  Agostino  Dott.  Gemelli,  O.M.1 

The  worthy  translator  has  inserted  a  long  introduction  in 
which  he  states  his  own  opinions  on  the  theory  of  evolution,3 
and  throughout  his  translation  he  has  inserted  many  remarks 
of  his  own.3 

The  Italian  edition,  therefore,  for  which  Gemelli  alone  is 
responsible,  is  in  many  respects  a  totally  new  work,  and  I 
trust  that  it  will  meet  with  as  friendly  a  reception  in  Italy 
as  that  accorded  to  the  German  edition  on  this  side  of  the 

I  am  deeply  grateful  to  all  my  colleagues  who,  by  supplying 
information  or  suggesting  additions,  have  helped  me  in  bring- 
ing out  this  new  German  edition  ;  and  I  am  especially  indebted 
to  Father  Eobert  de  Sinety  for  some  valuable  remarks  on  the 
most  recent  discoveries  regarding  the  problem  of  reduction 
in  Chapter  VI.  Father  H.  Muckermann,  S.  J.,  was  kind  enough 

1  La  biologia  moderna  e  la  teoria  delf  evohizione,  Florence,  1906. 

2  Gemelli  does  not  call  his  theory  the  theory  of  evolution,  but  prefers  to 
speak  of  polyphyletic  evolution  (Polifilogenesi).     As  I  also  have  expressed 
myself  in  favour  of  polyphyletic  evolution,  there  is  no   actual   discrepancy 
in  our  opinions,  although  I  have  retained  the  name  '  theory  of   evolution.' 
The  chief  difference  between  us  and  the  Monists  on  the  subject  of  evolution 
is  not  so  much  whether  it  is  polyphyletic  or  monophyletic,  but  it  affects 
rather  the  fundamental  principles  underlying  it,  for  we  accept  the  Christian 
cosmogony,  which  is  in  direct  opposition  to  that  of  Monism. 

3  These  remarks  are  in  many  cases  added  to  my  statements,  in  such  a  way 
as  to  make  it  difficult  to  decide  who  is  answerable  for  them.     This  remark, 
however,  does  not  apply  to  Chapter  X. 


to  lend  me  the  excellent  photographs  which  are  reproduced 
on  Plates  VI  and  VII  in  this  edition.1 



Feast  of  St.  Ignatius,  1906. 

1  These  and  many  other  original  photographs  have  been  prepared  by  Dr. 
Wm.  Gray  at  the  U.  S.  Army  Medical  Museum  in  Washington  for  his  new 
English  textbook  on  physiology,  that  will  shortly  be  published.  (Cf.  the 
list  of  plates  in  this  edition,  p.  xxxii.)  Any  other  reproduction  of  Plates  VI 
and  VII  is  forbidden. 


THESE  sketches  on  biology  and  the  theory  of  evolution 
appeared  in  book  form  barely  two  years  ago,  and  I  could 
hardly  expect  that  an  edition  of  2000  copies  would  be  so 
soon  exhausted.  My  friends  had  in  fact  told  me  bluntly 
that  the  book  was  too  dry  to  find  many  readers,  and  that 
it  made  too  great  demands  upon  the  power  of  thought 
possessed  by  our  educated  classes. 

It  is  true  that  the  book  has  not  sold  so  quickly  as  Haeckel's 
'  Eiddle  of  the  Universe,'  but  it  is  not  a  popular  scientific 
polemic  aiming  at  the  overthrow  of  Christianity,  and  there- 
fore peculiarly  welcome  to  those  lower  classes  which  are 
especially  interested  in  this  overthrow.  It  is  rather  an  attempt 
at  conciliation,  based  upon  an  objectively  scientific  foundation, 
and  it  aims  at  harmonising  the  ideas  of  modern  biology  with 
the  Christian  cosmogony,  and  thus  it  was  not  likely  to  prove 
acceptable  except  to  men  of  culture  and  intelligence.  Never- 
theless the  comparatively  quick  sale  of  the  book,  and  the 
numerous  discussions  to  which  it  has  given  rise,  show  that 
it  has  awakened  considerable  interest  among  educated  men 
in  Germany.1 

The  kind  of  interest  thus  awakened  varies  according  to 
the  personal  views  of  those  in  whom  it  exists.  They  may  be 
divided  into  three  classes,  viz.  (1)  supporters  of  Christianity, 
(2)  scientific  specialists,  and  (3)  opponents  of  Christianity. 
The  classification  is  not  quite  accurate,  because  there  are 
many  scientific  men,  and  especially  many  zoologists,  among 
the  readers  of  the  first  class,  and  among  those  of  the  third 
class  zoologists  form  a  considerable  majority.  Under  the 
second  category  I  include  those  only  who  confine  themselves 

1  Germany  is  here  used  to  include  Austria  and  all  countries  where  German 
is  spoken. 



to  considering  the  biological  contents  of  my  book,  without 
allowing  their  philosophical  pre-suppositions  to  transpire. 
Apart  from  some  few  expressions  of  opinion  on  points  of  minor 
importance,  the  book  has  been  very  favourably  received  by 
the  supporters  of  Christianity  in  Germany,  both  Catholic  and 
Protestant.  Some  have  even  described  it  as  a  '  rescue  from 
bondage,'  because  it  has  shown  the  right  tactics  to  adopt  in 
the  struggle  between  Christianity  and  the  monistic  doctrine 
of  evolution.  I  will  not  allude  further  to  the  various  reviews 
of  it  that  have  appeared  in  the  German  Catholic  papers.  In 
the  Reformation  of  February  26,  1905,  there  is  an  article 
entitled  *  Ein  Jesuitenpater  als  Anhanger  des  Darwinismus  ?  ' 
('  A  Jesuit  as  a  supporter  of  Darwinism  ?  ')  by  E.  Dennert,  a 
Protestant  reviewer,  well  known  as  an  opponent  of  Darwinism, 
who  expresses  his  complete  agreement  with  my  views  on  the 
subject  of  evolution.  Of  the  reviews  by  Catholic  writers  in 
other  countries,  I  will  mention  only  three  of  the  most  important. 
The  first  appeared  in  a  North  American  periodical,  The 
Review,  of  November  24,  1904,  and  the  reviewer's  opinions 
coincided  on  all  points  with  my  own.  The  second,  which  is 
very  thorough,  appeared  in  the  number  for  April  and  May 
1905  of  the  Spanish  Razon  y  Fe,  and  although  the  writer 
at  the  close  of  his  article  says  that  he  prefers  for  the  present 
to  abide  by  the  theory  of  permanence,  still  his  verdict  as  to 
the  author's  position  with  regard  to  the  theory  of  evolution 
is  favourable.  The  third  review,  *  L'Haeckelisme  et  les  idees  du 
Pere  Wasmann  sur  1'evolution,'  may  be  found  in  the  Belgian 
Revue  des  Questions  scientifiques  for  \  January  1906.  The 
French  critic,  himself  an  eminent  biologist,  in  the  course  of  a 
very  careful  article,  shows  that  it  is  not  possible  to  oppose  the 
monistic  doctrine  of  evolution  with  success,  unless  we  acknow- 
ledge the  claims  of  the  scientific  theory  of  evolution  ;  on  this 
point  he  agrees  fully  with  the  author's  opinions. 

Eeviews  written  by  critics  belonging  to  what  I  have  called 
the  second  class  deal  with  the  book  from  the  scientific  aspect. 
On  the  whole  they  are  appreciative  and  favourable,  although 
some  few  objections  have  been  raised.  I  will  mention  only 
the  articles  contributed  by  Professor  Dr.  C.  Emery  to  the 
BiologiscJies  Zentralblatt  (February  15,  1905)  ;  by  Dr.  E. 
Hanstein  to  the  Naturwissenschaftliche  Rundschau  (February 


2,  1905)  ;  by  J.  Weise  to  the  Deutsche  Entomologische  Zeit- 
schrift  (1905,  part  I)  ;  by  Dr.  K.  Holdhaus  to  the  Verhand- 
lungen  der  Zoologisch-botanischen  Gesellschaft  von  Wien  (1905, 
parts  5  and  6)  ;  and  by  Professor  H.  J.  Kolbe  to  the 
Naturwissenschaftliche  Wochenschrift  (July  2,  1905).i 

The  critics  of  the  third  class  are  those  who  seek  to  maintain 
their  own  monistic  theory  in  opposition  to  the  author,  and 
to  prove  his  position  as  a  Christian  untenable.  It  was  easy 
to  foresee  that  there  would  be  many  reviews  written  from  this 
standpoint,  as  unfortunately  most  of  the  zoologists  of  the 
present  day  have  monistic  tendencies  ;  and  the  fact  that  my 
book  called  forth  such  vigorous  opposition  may  be  regarded 
as  far  more  satisfactory  evidence  of  its  success  than  the  most 
appreciative  comments  proceeding  from  the  Catholic  party. 
Why  have  the  monists  thought  it  necessary  to  pay  so  much 
attention  to  my  work  ?  The  only  psychological  explanation 
of  their  action  is  that  they  see  in  it  a  certain  amount  of  danger 
to  the  supremacy  of  their  an ti- Christian  views.  For  this 
reason  they  do  their  best  to  draw  as  sharp  a  distinction  as 
possible  between  the  author  as  scientist  and  as  theologian. 
They  cannot  help  recognising  the  merits  of  the  book,  and 
the  only  objections  they  can  raise  refer  to  minor  points,  or 
are  based  on  misunderstandings  and  misrepresentations,  but 
naturally  they  refuse  to  acknowledge  that  the  author  has 
succeeded  in  reconciling  biology  in  its  recent  developments 
with  the  principles  of  Christianity,  for  such  an  acknowledge- 
ment would  at  once  deprive  modern  unbelief  of  one  of  its 
chief  weapons  in  the  conflict  with  Christianity. 

Of  these  hostile  criticisms  I  can  only  refer  here  to  the 
most  important,  those,  namely,  of  K.  Escherich,  H.  von 
Buttel-Reepen,  Ernst  Haeckel,  August  Forel,  J.  P.  Lotsy 

1  On  pp.  426  and  427,  where  Kolbe  has  attempted  to  give  a  summary 
of  the  '  results '  of  my  opinions,  there  are  some  misstatements,  that  are 
probably  due  to  some  extent  to  Escherich's  review,  to  which  reference  will  be 
made  later.  Kolbe's  fourth  point,  that  *  polyphyletic  origin  of  closely  allied 
forms  is  more  likely  than  monophyletic,'  is  exactly  the  opposite  of  my 
assertions.  The  remark  on  the  sixth  point  regarding  '  the  great  number  of 
primitive  types '  is,  to  say  the  least,  inaccurate.  The  statement  on  the  ninth 
point  that  the  assumption  of  a  '  creation  '  of  primary  types  is  '  a  dualism 
irreconcilable  with  the  principles  of  natural  science  '  is  devoid  of  all  proof. 
The  reviewer,  however,  seems  to  have  had  in  his  mind  some  notion  of  *  creation 
out  of  nothing,'  because  in  discussing  the  tenth  point  he  says  emphatically 
that  '  nevertheless  '  in  another  place  I  have  assumed  '  that  the  primary 
types  must  originally  have  been  formed  out  of  matter.' 


and  F.  von  Wagner.  They  are  not  all  written  in  the  same 
spirit,  as  the  following  examination  of  them  will  show. 

'  Kirchliche  Abstammungslehre  ' — the  Church's  teaching 
on  descent — is  the  title  of  a  long  article  by  Dr.  K.  Escherich, 
lecturer  on  zoology,  in  the  supplement  to  the  Allge- 
meine  Zeitung  of  February  10  and  11,  1905.  He  speaks 
very  appreciatively  of  my  position  with  regard  to  the 
theory  of  evolution,  and  especially  of  the  ninth  chapter,  in 
which  I  have  dealt  with  the  inquilines  or  guests  among  ants 
and  termites  from  this  point  of  view.  But,  on  the  other  hand, 
he  believes  that  '  theological  reasons  '  have  led  me  to  assume 
a  polyphyletic  evolution,  which  distinguishes  as  many  '  natural 
species  '  as  there  are  lines  of  evolution,  independent  of  one 
another,  and  he  thinks  that  I  have  done  this  in  order  the 
better  to  reconcile  the  doctrine  of  evolution  with  that  of 
creation.  My  opinions  regarding  the  origin  of  life  and  the 
creation  of  man  seem  to  him  inadmissible,  for  they  contradict 
the  most  important  postulates  of  the  monistic  doctrine  of 
evolution.  Escherich  sums  up  the  results,  which  he  thinks 
he  can  deduce  from  my  opinions,  and  arranges  them  under 
nine  chief  headings,  whence  he  draws  the  conclusion  '  that 
any  reconciliation  of  the  doctrine  of  descent  with  ecclesiastical 
dogmas  is  impossible.' 

My  reply  to  Escherich's  review  appeared  in  the  supplement 
to  the  Allgemeine  Zeitung  of  March  9,  1905.  In  it  I  showed 
that  the  reviewer's  imaginary  opposition  between  an  eccle- 
siastical and  a  non-ecclesiastical  doctrine  of  descent  indicated 
a  biased  misrepresentation  of  facts.  He  ought  to  have 
proved  that  the  doctrine  of  evolution  as  a  scientific  hypothesis 
and  theory  was  incompatible  with  the  Christian  cosmogony, 
but  instead  of  doing  so,  he  had  recourse  to  the  postulates  of  a 
monistic  philosophy,  which  are  neither  based  on  science  nor 
philosophically  correct.  I  drew  attention  also  to  a  number 
of  actual  misunderstandings  with  regard  to  the  *  natural 
species  '  and  the  '  inner  laws  of  evolution,'  &c.  These,  I 
believe,  were  accidental,  but  of  the  nine  points  which  Escherich 
ascribes  to  me  as  summing  up  my  opinions,  three  at  least  were 
wrongly  so  ascribed,  and  these  were  the  very  three  which  might 
have  been  challenged  from  the  scientific  standpoint. 

In  the  '  Closing  Word  '  appended  to  my  reply  by  Escherich, 


he  acknowledged  several  of  the  misunderstandings  as  such, 
but  he  adhered  to  his  assertion  that  my  doctrine  of  descent 
ought  to  be  described  as  '  illogical '  in  contrast  to  the  '  logical ' 
theory.  Unhappily  he  forgot  to  add  that  the  logical  character 
of  the  monistic  view,  which  he  maintains,  has  no  scientific 
basis,  but  rests  upon  the  unproved  postulates  of  a  false  philo- 
sophy. He  concluded  by  recommending  my  book  to  all 
readers  who  had  had  a  scientific  education,  but  warned  the 
general  public  against  reading  it !  I  am  grateful  to  him  for 
this  recommendation,  as  I  wrote  expressly  for  educated 

In  the  Archiv  fur  Rassen-  und  Gesellschaftsbiologie 
(March- April,  1905)  there  appeared  a  very  careful  criticism 
of  my  book,  contributed  by  Dr.  H.  von  Buttel-Eeepen,  who 
is  a  specialist  on  the  subject  of  social  insects.  The  review  is, 
on  the  whole,  written  in  a  friendly  spirit,  but  it  forces 
into  prominence  the  question  of  cosmogony.  '  Where  does 
science  end,  and  the  Jesuit  begin  ?  '  This  is  the  subject  for 
discussion.  The  '  science  '  which  the  book  contains  is  praised 
by  von  Buttel,  but  he  prefers  to  have  nothing  to  do  with 
'  that  web  of  inconsistency,  which,  solely  in  order  to  save  a 
number  of  dogmas,  draws  its  illogical  and  untenable  threads 
over  Wasmann's  scientific  work,  obscuring  the  results  of 
research.'  By  this  '  web  of  inconsistency '  he  means  my 
views  on  the  theory  of  creation,  on  spontaneous  generation,  and 
on  the  descent  of  man.  That  in  these  points  I  have  not  been 
'  consistent '  in  the  reviewer's  monistic  sense,  may  soothe  my 
conscience,  not  only  as  a  theologian,  but  also  as  a  scientific 
man  and  a  philosopher. 

By  means  of  his  lectures  at  the  Berlin  Singakademie 
(April  1905),  Professor  Ernst  Haeckel,  the  well-known  prophet 
of  Darwinism,  undoubtedly  did  very  much  to  increase  the 
circulation  of  my  '  Biology  and  the  Theory  of  Evolution.' 
Special  importance  may  be  attached  to  his  criticism,  as  he 
states  expressly,  both  in  the  preface  and  in  the  supplement 
to  the  printed  edition  of  his  lectures  on  the  theory  of  evolution, 
that  he  was  induced  to  deliver  them  chiefly  through  the  publica- 
tion of  my  book.  What  was  the  result  of  this  official  criticism, 
which  Haeckel  as  the  champion  of  German  monism  felt  bound 
/to  pronounce  ?  On  the  one  hand  he  welcomes  my  work  as  a 


satisfactory  proof  that  the  Catholic  Church  has  ceased  to 
oppose  the  doctrine  of  evolution,  and  on  the  other  hand  he 
calls  it  a  masterpiece  of  Jesuitical  distortion  and  sophistry. 
He  bestows  upon  it  the  highest  praise  that  could  proceed 
from  his  lips,  when  he  says  that  the  ninth  chapter  (The  Theory 
of  Permanence  or  the  Theory  of  Descent)  might  be  incorporated 
as  a  valuable  addition  in  one  of  Darwin's  works,  but  at  the 
same  time  he  regards  it  as  one  of  the  achievements  of  '  the 
marvellous  system  of  falsification  invented  by  the  Jesuits.' 
I  cannot  but  be  grateful  to  Haeckel  for  the  contradictory  elo- 
quence with  which  he  has  denounced  my  book  as  a  dangerous 
*  snare  '  for  all  who  are  not  yet  perfectly  convinced  monists, 
for  I  believe  that  his  very  denunciation  has  led  no  small  number 
of  victims  into  that  snare,  and  has  induced  them  to  read  the 
book  which  he  has  solemnly  placed  on  the  index  for  Monism. 

It  would  be  superfluous  for  me  on  this  occasion  to  discuss 
Haeckel's  statements  in  detail.  In  an  '  Open  Letter  to  Professor 
Haeckel,'  which  appeared  on  May  2, 1905  in  the  Germania  and 
in  the  Kolnische  Zeitung,  I  answered  his  assertions  clearly  and 

'  Wissenschaft  oder  Kohlerglaube  ?  '  (*  Science  or  charcoal- 
burner's  Faith  ? ')  is  the  title  of  an  article  antagonistic  to 
me,  that  appeared  in  the  Biologisches  Zentralblatt  for  1905, 
Nos.  14  and  15.  It  was  written  by  the  well-known  authority 
on  ants,  Professor  August  Forel.  He  does  not  discuss  ants 
in  this  article,  in  which  in  fact  he  pays  a  high  tribute  to  my 
scientific  knowledge,  but  he  challenges  my  '  charcoal-burner's 
faith/  by  which  he  means  my  energetic  defence  of  Christianity 
against  the  attacks  of  Monism.  Two  years  previously  I 
had  contributed  to  the  same  paper  (Nos.  16  and  17,  1903)  a 
calm  and  courteous  criticism  of  Forel's  monistic  theory  of 
identity,1  and  this  was  his  reply  to  it,  expressed  however  in 
by  no  means  the  same  appropriate  terms,  but  in  language 
that  showed  irritability,  occasionally  bordering  on  fanaticism. 
In  the  introduction  to  his  article  he  states  plainly  why  his 
reply  was  so  long  delayed,  and  why  it  displays  so  much  hostility; 
he  says  :  '  In  the  meantime  Wasmann  has  worked  out  and 
favoured  us  with  a  doctrine  of  descent  sui  generis.  .  .  .  Now 

1  See  my  Instinkt  und  Intelligenz  im  Tierreich,   Freiburg    im    Breisgau, 
1905,  3rd  edit.,  chap.  xii. 


that  Wasmann  is  beginning  to  be  the  apostle  of  a  new  doctrine,1 
I  regard  it  as  my  duty  to  answer  him.' 

Forel  was  therefore  annoyed  by  my  attempt  to  show  that 
the  theory  of  evolution  was  not  irreconcilable  with  Christianity, 
and  instead  of  impartially  disproving  my  opinions,  he  showed 
a  partisan  spirit  in  trying  to  distort  them,  and  allowed  his 
imagination  free  scope  in  ridiculing  the  *  natural  species,' 
whose  primitive  forms  I  assumed  to  have  been  created  by  God. 
His  charges  against  '  charcoal-burner's  faith,'  or  rather  against 
the  Christian  standpoint,  are  based  upon  a  confusion  of  ideas, 
such  as  one  would  hardly  expect  in  a  critic  who  has  been  .. 
trained  in  philosophy.  Finally,  to  crown  his  arguments,  he  *N 
ingeniously  makes  fun  of  the  letters  S.J.  (Societatis  Jesu) 
after  my  name  ;  he  says  S  stands  for  scientist  and  J  for  Jesuit, 
and  advises  me  to  put  an  end  to  the  unhappy  union  of  the 
two  letters.  He  goes  even  further  and  enlarges  upon  this 
distinction  in  the  following  words  :  '  Wasmann  S.  is  a  scientific 
man,  whom  I  respect  for  his  acumen  and  conscientious  work  ; 
Wasmann  J.  is  a  scholastic  Jesuit.  But  Wasmann  S.  is  a  slave 
under  the  control  of  Wasmann  J.,  and  can  be  free  and  inde- 
pendent only  when  he  deals  with  matters  on  which  he  does  not 
come  into  conflict  with  Wasmann  J.  As  soon  as  any  dispute 
arises,  Wasmann  S.  ceases  to  think  as  a  man  of  science  and 
Wasmann  J.  begins  with  his  syllogisms  and  scholasticism 
and  all  the  war  of  words.' 

Such  an  attack  did  not  really  require  any  answer  at  all, 
as  it  revealed  its  character  plainly  enough.  Nevertheless,  I 
wrote  a  short  article  in  reply,  entitled  '  Wissenschaftliche 
Beweisfiihrung  oder  Intoleranz  ?  '  ('  Scientific  Proof  or  In- 
tolerance ? ')  which  appeared  in  No.  18  of  the  Biologisches 
Zentraiblatt  for  1905.  I  had  no  difficulty  in  showing  that 
it  would  have  been  better  for  Forel  to  have  said  nothing 
than  to  have  come  forward  with  such  weapons  as  the  champion 
of  Monism. 

In  their  attacks  upon  my  book,  both  Haeckel  and  Forel 
have  had  many  followers  in  popular  scientific  circles  of  the 
same  tendency.  There  is  nothing  surprising  in  this  fact, 
and  it  does  not  call  for  any  further  comment. 

1  These  words  allude  to  my  lectures  on  evolution  delivered  in  Germany 
and  Switzerland. 



It  is  more  significant  that  Forel's  joke  about  Wasmann  S. 
and  Wasmann  J.  has  been  imitated  even  in  highly  learned 
university  lectures.1 

Lotsy  praises  the  author  of  '  Biology  and  the  Theory  of 
Evolution  '  very  highly,  and  says  :  '  Wasmann  is  a  Jesuit, 
but  at  the  same  time  he  is  one  of  the  best  zoologists  of  the 
present  day,  and  we  must  feel  the  deepest  admiration  for 
his  investigations  into  the  life  of  ants.  This  very  eminent 
man  writes  on  p.  271 :  "  Of  two  hypotheses  in  natural  science 
or  natural  philosophy,  put  forward  as  offering  an  explanation 
of  one  and  the  same  series  of  facts,  it  behoves  us  always  to 
choose  the  one  which  succeeds  in  explaining  most  by  natural 
causes,  and  on  this  principle  we  can  hardly  hesitate  to  choose 
the  theory  of  descent  in  preference  to  that  of  permanence." 
But  as  soon  as  we  have  to  consider  man.  .  .  .'  Lotsy  goes  on 
to  refer  to  p.  283  of  my  book,  where  I  have  limited  the  scope 
of  zoology  with  regard  to  man  to  his  body,  declaring  it  and 
its  attendant  sciences  incompetent  to  deal  with  him  on  his 
spiritual  side.  On  this  subject  Lotsy  remarks :  *  These 
words  remind  me  of  Lamarck's  saying,  "  Telles  seraient  les 
reflexions  que  Ton  pourrait  faire,  si  1'homme  n'etait  distingue 
des  animaux  que  par  les  caracteres  de  son  organisation,  et 
si  son  origine  n'etait  pas  differente  de  la  leur."  Are  we  to 
accuse  Wasmann  of  prevarication  ?  Certainly  not.  I  fully 
agree  with  what  Forel  said  a  few  days  ago  in  the  Biologisches 
Zentralblatt.  Forel  sees  in  Wasmann  two  distinct  person- 
alities, the  scientist  and  the  theologian,  whom  I  shall  designate 
by  A.  and  B.'  Then  follows  verbatim  Forel's  distinction  that 
I  have  already  quoted,  the  only  difference  being  that  for 
Wasmann  S.  and  Wasmann  J.,  Lotsy  writes  A.  and  B. 

Lotsy  might  easily  have  perceived  the  weakness  of  this 
argument  of  Forel's,  if  he  had  really  considered  the  passage 
quoted  from  Lamarck,  who  agrees  with  me  in  declaring  zoology 
alone  incompetent  to  deal  with  the  question  of  the  origin  of 
man.  If  Lotsy  were  consistent,  he  would  have  to  see  two 
personalities,  viz.  a  scientific  man  and  a  '  scholastic  Jesuit,'  in 
Jean-Baptiste  Pierre  Antoine  de  Monet,  Chevalier  de  Lamarck ! 

1  J.  P.  Lotsy,  Vorlesungen  uber  Deszendenztheorien,  mit  besonderer  Beriick- 
sichtigung  der  botanischen  Seite  der  Frage  ('  Lectures  on  theories  of  descent, 
with  especial  reference  to  the  botanical  side  of  the  question  '),  at  the  Imperial 
University  of  Leiden,  Part  I,  Jena,  1906,  pp.  328,  329. 


Special  reference  is  due  to  a  very  detailed  criticism  of  ray 
book  that  appeared  in  the  Zoologisches  Zentralblatt,  a 
scientific  periodical  (1905,  No.  22).  The  review  was  written 
by  F.  von  Wagner  of  Giessen,  professor-extraordinary  of 
zoology,  yet  it  is  not  of  a  purely  scientific  character,  but 
shows  a  partisan  spirit,  although  the  author's  anti-Christian 
bias  is  not  so  bluntly  expressed  as  is  the  case  in  Haeckel's  and 
Forel's  articles.  It  is,  however,  perceptible  throughout  the 
review,  which  is  consequently  quite  unlike  the  impartial 
criticisms  that  we  usually  find  in  the  Zoologisches  Zentralblatt. 

In  the  introduction  to  the  nine  pages  in  which  he  deals 
with  my  book,  von  Wagner  remarks  that  not  a  few  of  his 
fellow-zoologists  have  been  induced  to  believe  that  Wasmann's 
attitude  towards  the  theory  of  evolution  indicates  a  '  change 
of  front  on  the  part  of  the  Catholic  Church  with  regard  to 
modern  biology.'  The  reviewer  does  his  best  to  deliver  his 
colleagues  from  this  *  illusion,'  and  I  am  grateful  to  him  for 
doing  so,  as,  like  Haeckel  and  Forel,  von  Wagner  does  not 
mean  by  '  modern  biology  '  merely  its  scientific  results,  but 
also  the  monistic  postulates  which  the  opponents  of  Christianity 
have  insisted  upon  attaching  to  these  results.  I  gladly  agree 
with  the  reviewer,  and  confess  that  my  views  do  not  coincide 
with  the  postulates  of  a  false  philosophy,  by  no  means  free 
from  hypotheses.  This  is,  however,  all  that  he  has  really 
succeeded  in  proving. 

Von  Wagner  himself  acknowledges  that  within  my  own 
field  of  research  I  '  apply  the  principles  of  evolution  in  a 
scientific  spirit'  (p.  691),  and  he  describes  my  account  of 
modern  cytology,  or  the  study  of  cells,  from  the  scientific 
standpoint  as  'very  successful'  (p.  693).  He  is,  moreover, 
particularly  '  grateful '  for  those  parts  of  the  book  which 
contain  '  an  excellent  summary  of  the  important  results  of 
Wasmann's  investigations  from  the  standpoint  of  the 
principle  of  descent/  The  historical  account,  too,  of  the 
development  of  biology  '  describes  it  accurately  in  its  general 

We  must  now  consider  the  reviewer's  objections,  which 
can  be  summed  up  in  one  sentence  (p.  692)  :  '  The  book  in 
question  has  one  author,  but  two  editors,  a  scientific  man 
engaged  in  research  work  and  a  theologian.  Consequently, 



the  whole  is  a  joint  production  ;  the  theologian  takes  the 
lead,  and  the  scientific  man  may  assert  himself  only  so  far  as 
the  former  gives  permission.'  The  conclusion  derived  by 
von  Wagner  from  this  statement  is  that  the  book  is  written 
with  a  bias  from  beginning  to  end. 

The  answer  to  this  is  obvious  ;  we  need  only  apply  the 
just  quoted  words  of  the  reviewer  to  his  own  review.  *  The 
review  in  question  has  one  author,  but  two  editors,  a  scientific 
man  engaged  in  research  work  and  a  monistic  philosopher. 
Consequently,  the  whole  is  a  joint  production  ;  the  monistic 
philosopher  takes  the  lead,  and  the  scientific  man  may  assert 
himself  only  so  far  as  the  former  gives  permission.'  The 
conclusion  that  we  derive  from  this  statement  is  that  the 
review  is  written  with  a  bias  from  beginning  to  end. 

Let  us  now  examine  my  book  more  closely  and  see  how  far 
the  '  bias  '  imputed  to  it  by  the  reviewers  really  exists,  and 
how  far  they  are  mistaken. 

Even  in  my  account  of  the  historical  development  of 
biology  von  Wagner  discovers  a  bias,  for  he  says  that  I  have 
singled  out  for  praise  none  but  Christian  representatives  of 
this  science.  I  do  not  understand  why,  if  this  were  the  case, 
I  spoke,  as  he  says,  with  remarkably  scant  appreciation  of 
Cuvier's  achievements  in  comparative  anatomy,  and  men- 
tioned Bichat's  work  in  more  eulogistic  terms,1  whereas  if 
my  opinion  were  really  biased,  I  should  have  extolled  Cuvier 
rather  than  Bichat,  as  being  an  eminent  Christian  as  well  as  a 
scientific  man.  This  fact  shows  that  von  Wagner's  desire  to 
discover  a  particular  bias  in  my  work  is  the  outcome  of  his 
own  imagination. 

The  bias  of  the  book,  as  von  Wagner  has  discovered  (p.  694), 
is  revealed  especially  '  in  what  it  does  not  contain.'  The 
author  is  accused  of  having  purposely  withheld  from  his  readers 
the  more  general  biological  evidence  in  favour  of  the  theory 
of  evolution.  I  feel  inclined  to  ask  whether  the  reviewer  has 
really  read  the  eighth  and  ninth  chapters  of  his  edition.  I  am 
supposed  not  to  have  referred  to  Darwin,  Lamarck  and  Geoffroy 
St.  Hilaire,  whereas  they  are  all  mentioned  on  p.  169.  He 
seems  not  to  have  noticed  the  more  general  relations  of  the 

1  In   speaking  thus  I  relied  upon  M.  Duval's  statements  in  his  Precis 
d'histologie,  a  book  with  which  von  Wagner  seems  not  to  be  acquainted. 


theory  of  evolution  to  the  Copernican  theory  of  the  universe, 
to  modern  geology  and  palaeontology  (pp.  179-85),  and  the 
long  dissertation  following  them  on  the  limits  and  causes  of 
the  hypothetical  phyletic  evolution,  but  he  notices  my  state- 
ments regarding  '  natural  species  '  and  their  connexion  with 
the  theory  of  creation,  for  these  statements  give  him  another 
opportunity  of  joining  Escherich,  Haeckel  and  Forel  in  imput- 
ing to  me  a  theological  bias.  On  pp.  219,  220,  I  referred 
expressly  to  the  mass  of  indirect  evidence  supporting  the 
theory  of  evolution  to  be  derived  '  from  comparative  morpho- 
logy, comparative  history  of  evolution,  comparative  biology 
and  especially  from  palaeontology,'  but  I  said  that  I  had  no 
intention  on  this  occasion  of  writing  a  textbook  of  the  theory 
of  descent.  'Ko  one  could  discover  in  this  any  intentional 
concealment  of  evidence,  who  did  not  wilfully  misinterpret 
my  words  by  imputing  to  them  a  bias  that  is  not  there.  Such 
a  critic  is  plainly  incapable  of  forming  a  just  and  objective 

Let  us  for  a  moment  regard  the  matter  from  the  point  of 
view  of  an  extreme  supporter  of  the  theory  of  permanence. 
He  would  have  quite  as  much  justification  for  discovering  a 
bias  in  favour  of  the  theory  of  evolution  from  those  very 
statements  and  omissions,  in  which  a  fanatical  advocate  of 
the  theory  discovers  a  bias  hostile  to  it.  He  might,  for  in- 
stance, try  to  account  for  the  fact  that  I  have  not  discussed 
in  detail  the  ordinary  evidence  in  favour  of  the  theory  of 
evolution,  by  declaring  that  this  evidence  has  lost  most  of  its 
weight  through  Fleischmann's  criticism,  and  therefore  I  have 
been  obliged  to  establish  the  scientific  justification  of  the 
evolution  hypothesis  upon  the  new  and  independent  basis  of 
my  own  research.  Moreover,  when  I  have  expressed  my 
preference  for  '  natural  species '  rather  than  '  systematic 
species,'  he  might  discover  an  intention  to  set  aside  the  theory 
of  permanence  and  replace  it  by  that  of  evolution,  under  the 
pretext  that  the  latter  is  more  easily  reconciled  with  the 
Christian  doctrine  of  creation,  &c.  I  maintain,  therefore, 
that,  where  it  is  possible  to  see  in  the  same  statements  of  any 
author  two  totally  opposite  tendencies,  it  is  plain  that  both 
imputations  are  alike  objectively  without  foundation.  I 
need  say  no  more  regarding  von  Wagner's  method  of  treating 


my  book,  as,  whilst  imputing  a  biased  tendency  to  me,  he 
shows  the  same  himself. 

I  must  acknowledge  that  with  regard  to  the  doctrine  of 
creation,  the  hypothesis  of  spontaneous  generation  and  the 
implication  of  the  theory  of  descent,  I  had  a  bias,  and  one 
that  is  directly  opposed  to  that  of  my  reviewer.  I  had  the 
intention  of  proving  that  a  reasonable  theory  of  evolution 
necessitates  our  assuming  the  existence  of  a  personal  Creator, 
and  1  wrshed  further  to  show  that  '  spontaneous  generation  * 
was  scientifically  untenable,  and,  therefore,  could  not  be  a 
postulate  of  science.  Finally,  I  desired  to  prove  that  toregard 
man  from  the  purely  zoological  point  of  view  is  a  one-sided  and 
1  was,  however,  forced  to  adopt  this 

threefold  bias  by  the  monists,  who  were  exerting  themselves 
with  a  much  greater  bias  to  establish  false  philosophical 
postulates  in  the  name  of  biology,  and  to  force  them  as  'monistic 
dogmas  '  upon  all  interested  in  science.  I  considered  it  my 
duty  as  a  Christian  and  as  a  scientific  man  to  protest  vigorously 
against  these  attempts  at  a  fresh  subjugation  of  the  human 

It  is,  moreover,  psychologically  very  interesting  to  observe 
how  a  reviewer,  himself  an  ardent  advocate  of  Monism,  seeks 
to  discover  throughout  my  book  Christian  tendencies,  in  order 
to  destroy  as  far  as  possible  its  scientific  objectiveness.  A 
criticism  undertaken  on  these  lines  cannot  be  truly  free  from 
prejudice,  and  the  absolutely  biased  character  of  von  Wagner's 
review  appears  most  plainly  in  his  closing  words  (p.  699)  : 
'  There  is  always  the  same  discord,  when  science  is  only  on  a 
man's  lips  and  not  in  his  heart.'  Because  I  do  not  accept  the 
unscientific  postulates  of  Monism,  all  love  of  science  is  to  be 
denied  me  !  Is  not  that  plainly  monistic  intolerance  ?  Accord- 
ing to  my  opinion,  science  has  its  abode  neither  on  the  lips 
nor  in  the  heart,  but  in  the  intellect  or,  as  von  Wagner  would 
say,  the  brain,  which  he  regards  without  doubt  as  the  real 
organ  of  thought  in  a  human  being. 

And  now  I  take  leave  of  my  critics,1  and  commend  the 
present  edition  to  their  kind  attention.  In  it,  as  far  as  lay  in 

1  A  short  reply  to  von  Wagner's  review  has  already  appeared  in  Beispiele 
rezenter  Artenbildung  bei  Ameisengdsten  und  Termitengasten  (written  in 
honour  of  J.  Rosenthal,  Leipzig,  1906,  pp.  45-58  ;  Biologisches  Zentralblatt, 
1906,  Nos.  17  and  18,  pp.  565-580),  55  (577)  et  seq. 


my  power,  I  have  taken  into  account  all  the  really  well-founded 
objections  to  statements  in  the  previous  editions,  whether 
these  objections  were  raised  by  friends  or  by  opponents.  It  is 
in  vain,  however,  to  call  upon  me  to  conform  to  the  tyrannical 
requirements  of  Monism,  and  such  a  demand  will  remain 
unsatisfied  in  the  future,  as  it  has  done  in  the  past. 


(^4  more  detailed  outline  of  contents  is  prefixed  to  each  chapter) 


PREFACE  TO  THE  SECOND  EDITION    ......          v 

PREFACE  TO  THE  THIRD  EDITION     _.         .         .         .         .         .        vii 

A  FEW  WORDS  TO  MY  CRITICS          .         .         .         ...         .         xi 


Introduction    ..........  1 

1.  Meaning  and  subdivisions  of  Biology.     Tree  of  the  biological 

sciences     ..........  3 

2.  Earliest  development  of  Biology.     Aristotle.     Albert  the  Great. 

Roger  Bacon      .........  8 

3.  Development  of    systematic  zoology  and    botany.     Linnaeus' 

'  System  a  naturae  '  and  modern  syst      atics  .          .          .          .         17 



1.  Development  of  anatomy  before  the  nineteenth  century      .          .  25 

2.  Early  history  of  cytology         .......  29 

3.  Methods  of  staining  and  cutting  sections  .....  34 

4.  Use  of  the  microscope  in  studying  the  anatomy  and  ontogeny 

of  Termitoxenia  and  other  inquilines  amongst  ants  and  termites        37 

5.  Recent   advance   in   microscopical   research.        Cytologists   of 

various  nationalities    ........         45 


1.  The  cell,  a  mass  of  protoplasm  with  one  or  more  nuclei.    Diver- 

sity in  shape  and  size  of  cells,  and  in  number  of  nuclei        .         .         48 

2.  Structure  of  the  cell  examined  more  closely.     Theories  regarding 

the  structure  of  spongioplasm        ......         54 

3.  Minute    structure    of   the    nucleus.      Chemical    and    physical 

theories 60 

4.  Survey  of  the  historical  development  of  cytology        ...         63 





1.  The  living  organism  as  a  cell  or  an  aggregation  of  cells.    Processes 

of  life  involving  movement         ...  ...         66 

2.  Activity    of    living    protoplasm.     Amoeboid    movements    of 

Rhizopods  and  Leucocytes  ...  .  70 

3.  Exterior  and  interior  products  of  the  cell.     Various  biochemical 

departments  of  work  .......         74 

4.  Predominance  of  the  nucleus  in  the  vital  activities  of  the  cell. 

Vivisection  of  unicellular  organisms       .....         77 


1.  Various  kinds  of  division  of  the  cell  and  nucleus.     Direct  and 

indirect  nuclear  division   ......  .85 

2.  Stages  of  indirect  nuclear  division  (karyokinesis  or  mitosis)        .         88 

3.  Survey  of  the  process  of  karyokinesis.     Centrosomes  .          .         97 


(Plates  I  and  II) 

Introductory  remarks  .          .          .          .  .          .          .104 

1.  The  problems  to  be  solved 

2.  Maturation-divisions  of  the  germ-cells       .  ...       109 

3.  Normal  process  of  fertilising  an  animal  ovum     .  .119 

4.  Phenomena  of  superfecundation  among  animals,  and  double- 

fertilisation  in  plants.     Polyembryony  .          .          .          .127 

5.  Processes   of  conjugation  in  unicellular  organisms  and   their 

relation  to  the  problem  of  fertilisation    .          .          .          .  .130 

6.  Natural  parthenogenesis        .          .          .  •                .  .135 

7.  Artificial  parthenogenesis       .          .          .          .          .          .  .       139 

8.  Fertilisation  of  non-nucleated  egg-fragments  (merogony)  .       149 

9.  Review  of  the  subject  of  fertilisation  and  conclusions.  The 

essential  feature  of  fertilisation.  Twofold  purpose  of  fertilisa- 
tion. Chromosomes  as  bearers  of  heredity.  Mendel's  Law. 
Amphimixis.  Interior  laws  of  development  governing  organic 
life ...  155 


1.  The  cell  as  the  lowest  unit  in  organic  life.     Are  there  any  living 

creatures  whose  organisation  is  more  simple  than  that  of  the 
cell  ?  The  idea  of  individuality  in  unicellular  and  multi- 
cellular  organisms.  All  so-called  '  lower  elementary  units  '  in 
the  cell  have  no  real  existence  .  .  .  .  .  .179 

2.  Spontaneous    generation    of    organisms.     Untenable    character 

of  the  theories  of  spontaneous  generation  demonstrated  by 
modern  biology.  Theory  of  Creation  a  postulate  of  science  .  193 

CONTENTS  xxvii 



1.  The  problem  of  determination  and  its  history          .          .         .       209 

2.  More  detailed  discussion  of  the  problem  of  determination  .          .       218 

3.  Embryological  experiments  on  the  eggs   of  various   kinds  of 

animals     ..........       228 

4.  Conclusions.     Inadequacy  of  the  Machine  Theory.     Vitalistic 

solution  of  the  Problem  of  Life    ......       235 


1.  Problem  of  phylogeny  .......       251 

2.  Four  different  meanings  of  the  word   '  Darwinism.'     Critical 

remarks  upon  them  .          .          .          .          .          .          .  256 

3.  The  subject  of  the  theory  of  evolution  as  a  scientific  theory : 

investigation  of    facts  and  causes  with    reference   to    series 

of  organic  forms         ........       267 

4.  Theory  of  evolution  considered  in  the  light  of  the  Copernican 

theory  of  the  universe.     Biological  evolution  a  natural  con- 
sequence of  geological  evolution  .....       272 

5.  Philosophical  and  scientific  limitations  of  the  theory  of  evolution. 

First:  Philosophical  limitations.  Recognition  of  a  personal 
Creator.  His  influence  upon  the  origin  of  primitive  organ- 
isms. Creation  of  the  human  mind. 

Second :  Scientific  limitations.  Hypothesis  and  theory. 
Monophyletic  or  polyphyletic  evolution  ?  Problems  still 
to  be  solved  regarding  the  course  and  causes  of  the  hypo- 
thetical evolution  of  a  race  ......  279 

6.  Systematic  and  natural  species.     Importance  of  this  distinction 

from  the  point  of  view  both  of  natural  science  and  of  philo- 
sophy.    Theory  of  evolution  and  the  doctrine  of  Creation      .       296 

7.  Summary  of  results      ........       302 


(Plates  III— V) 

1.  Reasons  for  the  fixity  of  systematic  species     ....       307 

2.  Direct  evidence  in  support  of  the  theory  of  evolution.     Muta- 

tion and  cross-breeding  as  factors  in  forming  species      .         .       312 

3.  Evolution  of  the  forms  of  Dinarda.   Conclusions  drawn  from  it.       315 

4.  Indirect  evidence  in  support  of  the  theory  of  evolution  derived 

from  the  comparative  morphology  and  biology  of  inquilines 
amongst  ants  and  termites   .......       327 

5.  Hypothetical  phylogeny  of  the  Lomechusa  group.     Origin  of  its 

genera  and  species  through  the  action  of  natural  laws  of 
evolution  .........       330 

6.  Inquilines  amongst  the  wandering  ants.     Their  mimetic  char- 

acteristics.    Comparison  between  Dorylinae  inquilines  of  the 
mimetic  and  of  the  offensive  types,  and  the  Atta  inquilines    .       340 

xxviii  CONTENTS 


7.  Transformation   of    wandering    ants'    inquilines    into  termite- 

inquilines.  Recent  confirmation  and  extension  of  this  hypo- 
thesis ..........  348 

8.  The  family  of  Clavigeridae  ;    their  characteristics  prove,  when 

considered  from  the  point  of  view  of  evolution,  to  be  all  due 

to  adaptation      .          .          .          .          .          ....       360 

9.  The  hypothetical  phylogeny  of  the  Paussidae.     Adaptation  to 

more  complete  guest-relationship  has  been  the  principle 
controlling  their  evolution  .  .  .  .  .  .  .364 

10.  The  Termitoxeniidae,  a  family  of  Diptera.     Their  descent  from 
genuine  Diptera  may  be  proved  from  their  adaptation  char- 
acteristics and  the  development  of  their  thoracic  appendages      379 

11.  The  history  of  slavery  amongst  ants.     Survey  of  the  biological 

facts  upon  which  it  is  based.     Conclusions    ....       386 

12.  Conclusions  and  results.     Theories  of  permanence  and  descent 
compared  with  regard  to  their  value  in  supplying  explanations. 
The  latter  alone  can  suggest  natural  causes  to  account  for  the 
occurrence  of  beneficial  adaptations,  and  therefore  it  reveals 
the  Creator's  wisdom  and  power  more  strikingly  than  does 

the  theory  of  permanence    .......       425 


(Plates  VI  and  VII) 

Preliminary  observations.     Great  importance  of  this  question        .       431 

1.  Is  a  purely  zoological  view  of  man  justifiable  ?     Inadequacy 

of  such  a  view.  What  are  we  to  understand  by  the  creation 
of  man  ?  St.  Augustine  on  this  subject.  Philosophical 
reflexions  on  the  creation  of  man.  How  far  zoology  is  com- 
petent to  investigate  the  origin  of  man  .....  432 

2.  What  actual  evidence  is  there  of  the  descent  of  man  from 

beasts  ? 443 

(a)  A  glance  at  the  comparative  morphology  of  man  and 
beasts.  Wiedersheim's  *  testimony.'  Skeletons  of  apes 
and  men.  Rudimentary  organs  .....  443 
(6)  The  biogenetic  law  and  its  application  to  man.  Haeckel's 
progonotaxis  of  man.  Criticism  of  the  biogenetic  law  in 
itself  and  in  its  application  to  man  .....  446 

(c)  The  theory  of  direct  relationship  between  man  and  the 
higher  apes.     Their  '  blood-relationship '  .  456 

(d)  Theory   of   remote    community   of   origin   between   man 

and  apes.     Palseontological  arguments  against  this  theory.       462 

3.  Criticism  of    recent  palseontological  and    prehistoric  evidence 

for  the  descent  of  man  from  beasts         ....       465 

(a)  Pithecanthropus  erectus,  a  genuine  ape      ....       465 

(b)  The  Neandertal  man  and  his  contemporaries.     Schwalbe's 
theory  regarding  Homo  primigenius.     Recent  investigations 
by  Macnamara  and  Kramberger.     Homo  primigenius  merely 

an  early  species  of  man.     Homo  sapiens     ....       467 

(c)  Conclusions.  Haeckel's  imaginary  pedigree  of  the  Primates. 
Branco's  opinion  respecting  the   '  ancestors '   of  man.     A 
glance  into  the  future      .          .          .          .          .          .          .       476 





The  rock  of  the  Christian  cosmogony  amidst  the  waves  of  the 
fluctuating  systems  evolved  by  human  science.  The  storms  at 
the  base  of  the  rock  300  years  ago,  and  at  the  present  time. 
The  rock  can  never  be  overthrown  by  the  tempests,  because 
no  real  contradiction  between  knowledge  and  faith  can  ever 
exist 481 

(Plate  VIII) 

Lectures  on  the  Theory  of  Evolution  and   Monism,  delivered  at 

Innsbruck  in  October  1909 484 

SUPPLEMENTARY  NOTES    .         .         .         .         .         .         .523 

INDEX  525 



1.  Scheme  for  a  series  of  sections  of  Termitoxenia  (original)    .         42 

2.  Cells  of  various  shapes,  occurring  in  Termitoxenia  (original)     .         50 
3-6.  Diagrams    showing    the    historical    development    of     our 

knowledge  of  the  structure  of  cells  (after  Schlater)  .          .         64 

7.  Experimental     division    of    an     Infusorian    (Stentor)    (after 

Balbiani) <-„;     .          .         81 

8.  Direct  nuclear  division  of  the  red  blood-corpuscles  (after  Duval)         87 
9-12,  13-16.  Various     stages     of     indirect     nuclear     division 

(karyokinesis)  (after  Wilson)  .  .  .  .91  and  95 
17-22.  Diagrams  of  the  maturation-divisions  and  formation  of 

polar  bodies  in  the  egg-cell  (original)  .  .  .  .118 
23.  Transverse  section  of  an  embryo  of  Ascaris  megalocephala 

var.  bivalens,  at  the  blastula  stage  (original)  .  .  124 

24-26.  Pluteus  larvae  of  Echinus  and  Sphaer echinus,  and  of  their 

hybrid  (after  Boveri  from  Korschelt  and  Heider)      .          .       151 

27.  Position  of  the  spindles  in  an  Ascaris  egg  (after  Zur  Strassen)      223 

28.  Position  of  the  spindles  in  a  very  large  Ascaris  egg  (after  Zur 

Strassen) 224 

29.  Dinarda  Maerkeli  Ksw.  (original)          .....       316 

30.  Dinarda  dentata  Grav.  (original)  .....       316 

31.  Dinarda  Hagensi  Wasm.  (original)         .          .          .          .          .316 

32.  Dinarda  pygmaea  Wasm.  (original)         .          .          .          .          .316 

33.  Lomechusa  strumosa  F.  (original)  .          .          .         .          .       331 

34.  Larva  of  Lomechusa  strumosa  (original)  .          .          .          .331 

35.  Atemeles  pratensoides  Wasm.   being  fed  by  Formica  pratensis 

Deg.  (original  photograph)    .          ...          .          .         .  336 

36.  Mimeciton  pulex  Wasm.  (original)         .          .          .  .341 

37.  Ecitophya  simulans  Wasm.  (original)     .....  341 

38.  Xenocephalus  limulus  Wasm.  (original)            ....  344 

39.  Doryloxenus  Lujae  Wasm.  (original)        .....  344 

40.  Doryloxenus  transfuga  Wasm.  (original)         ....  353 

41.  Discoxenus  lepisma  Wasm.  (original)    .         .          .          .          .  353 

42.  Termitodiscus  Heimi  Wasm.  (original)           .          .          .          .  353 

43.  Pygostenus  pubescens  Wasm.  (original)           ....  357 

44.  Pygostenus  termitophilus  Wasm.  (original)   ....  357 

45.  Worker  of  Formica  sanguinea  Ltr.  (original  photograph)         .  394 



46.  (a)  Head  of  Formica  sanguinea  Ltr.        .....       398 

(6)  Head  of  Polyergus  rufescens  Ltr.  (original  photographs)      .       398 

47.  Ergatoid      queen      of      Polyergus      rufescens     Ltr.    (original 

photograph) 399 

48.  Worker  of  Polyergus  rufescens  Ltr.  (original  photograph)  .       399 

49.  Worker     of     Strong  ylognathus     testaceus     Schenk      (original 

photograph)         ........  403 

50.  Female  of  W heeleria  Santschii  For.  (original  photograph)          .  406 

51.  Male  of  Anergates  atratulus  Schenk  (original  photograph)         .  408 

52.  Cranium  of  the  Neandertal  man  (after  Schaafhausen)      .          .  468 

53.  Outline  of  the  sagittal  median  curve  : 

I.  Of  the  cranium  of  a  modern  Englishman 
II.  Of  the  cranium  of  a  modern  Australian  black 

III.  Of  the  Neandertal  cranium 

IV.  Of  the  Pithecanthropus  cranium 
V.  Of  the  chimpanzee  skull 

(after  Macnamara)          ....          .  469 

54.  Outline  of  the  sagittal  median  curve  : 

I.  Of  a  brachycephalic  Lapp  cranium 
II.  Of  a  dolichocephalic  Australian  cranium 
III.  Of  the  Neandertal  cranium 

(after  Macnamara)         ......       469 


At  the  End  of  the  Book 


I.  Diagrammatic    representation  of    the  process  of  fertilising 
an  egg-cell  (after  Boveri)  (printed  in  colours) 

To  illustrate  pp.  121-127 

II.  The  Chromosome  theory  and  Mendel's  Laws  (after  Heider) 

(printed  in  colours)  .         .         .         .To  illustrate  pp.  172,  173 

III.  Doryloxenus  transfuga,  Claviger  testaceus,  Pselaphus  Heisei, 

Paussiger  limicornis  and  Miroclaviger    cervicornis  (from 
original  photographs)         .         .          .To  illustrate  pp.  348-364 

IV.  Various  species   of  Paussidae  (from   original   photographs) 

To  illustrate  pp.  364-379 

V.  Termitophile    Diptera  of  the    Family    of    T ermitoxeniidae 

(from  original  photographs)  To  illustrate  pp.  37-44  and  379-386 

VI.  (a)  Skeleton  of  a  man.  (6)  Skeleton  of  an  ape  (orang- 
outang) (after  original  photographs  by  Dr.  Wm.  Gray, 
see  Preface,  p.  ix)  .  .  .To  illustrate  pp.  445  and  462 

VII.  (a)  Skull  of   a   man.     (b)  Skull  of  an   ape   (orang-outang) 

(after  original  photographs  by  Dr.  Wm.  Gray,  see  Preface, 

p.  ix)      .          .  .          .       To  illustrate  pp.  445  and  462 

VIII.  Human   skull    found    at    le  Moustier   (after   Hauser    and 

Klaatsch) To  illustrate  p.  511 






'  Knowledge  is  inexhaustible  in  its  source,  unlimited  by  time  or  space  in  its  force,  immeasurable 
in  its  extent,  endless  in  its  task,  unattainable  in  its  aim.' — K.  B.  V.  BAER. 


Biology  in  the  wider  and  narrower  signification  (p.  3).  Subdivisions  of 
Biology  (p.  4).  Tree  of  the  biological  sciences  and  its  branches 

(p.  5). 


Aristotle  as  the  father  of  the  biological  sciences  (p.  9).  Albert  the 
Great,  the  most  prominent  student  of  natural  science  in  the 
Middle  Ages  (p.  11).  Roger  Bacon  (p.  16). 


Linnaeus'  'Systema  naturae'  the  basis  of  modern  systematic  classifica- 
tion (p.  18).  The  most  recent  works  on  systematic  science  (p.  21). 
The  place  of  systematics  in  biology  (p.  24). 

AT  the  close  of  any  considerable  epoch  it  is  of  peculiar 
interest  to  look  back  upon  the  historical  development  of 
nations  and  states  during  that  period  ;  to  compare  their 
position  a  century  ago  with  that  which  they  now  occupy  ; 
to  observe  the  rise  and  fall  of  their  political  power,  and  the 
fluctuations  in  their  political  and  intellectual  importance 
amidst  the  pressure  of  contemporary  events,  and  to  trace  the 
causes  of  these  fluctuations.  In  the  same  way  it  is  most 
interesting  at  this  juncture  to  look  back  at  the  development 
of  a  science.  The  history  of  science  is  a  branch  of  universal 
history,  not  indeed  accompanied  by  the  thunder  of  cannon, 
like  the  great  battles  of  the  world,  but,  in  spite  of  its  silent 
working,  it  sometimes  has  more  influence  than  war  upon 
the  destiny  of  nations  and  of  humanity  as  a  whole. 


No  one,  I  think,  would  deny  that  during  the  past  century 
the  development  of  chemistry  and  physics,  and  of  the  technical 
arts  depending  upon  them,  has  been  of  the  utmost  importance 
in  advancing  the  growth  of  civilised  nations,  and  so  has  played 
no  small  part  in  the  history  of  the  world.  Modern  physics  have 
enabled  men  to  avail  themselves  of  the  forces  of  fire  and 
water,  and  the  discovery  of  steam  power  has  altered  the  face 
of  the  earth,  for  now  it  is  covered  with  a  network  of  railway 
lines,  upon  which  trains  rush  to  and  fro,  whilst  the  sea  too  is 
constantly  traversed  by  sea  monsters  built  of  steel  and  driven 
by  steam,  which  bring  the  farthest  ends  of  the  world  into 
communication,  and  convey  to  still  uncivilised  nations  the 
achievements  of  modern  progress.  By  means  of  physics,  too, 
has  the  human  intellect  succeeded  in  subjugating  the  mysterious 
waves  of  ether,  both  visible  and  invisible,  and  now  through 
the  electric  light  we  have  new  suns  ;  electric  telegraphs  and 
submarine  cables  have  triumphed  over  the  old  limitations  of 
time  and  space,  while  Kontgen-rays  penetrate  even  the  human 
body,  and  fix  the  outline  of  its  skeleton  on  photographic  plates. 
The  development  of  physics  and  chemistry  has  enabled  men 
to  construct  innumerable  motors  and  machines,  and  to  devise 
chemical  compounds  used  in  various  branches  of  industry, 
resulting,  on  the  one  hand,  in  a  complete  revolution  in  the 
economical  conditions  of  the  people,  and,  on  the  other  hand, 
supplying  our  armies  with  terrible  guns  and  deadly  explosives, 
in  the  invention  and  perfection  of  which  each  nation  strives 
to  outstrip  its  neighbours,  in  order  to  annihilate  them  more 
speedily,  should  an  opportunity  occur. 

It  is  obvious  that  astronomy  and  biology  owe  very  much 
to  their  kindred  science — physics,  and  especially  to  optics 
and  mechanics,  without  which  the  extraordinary  progress 
made  in  recent  times  would  have  been  impossible.  Optics 
and  mechanics  have  supplied  the  astronomer  and  the  biologist 
with  their  instruments,  and,  in  conjunction  with  chemistry, 
have  given  them  technical  methods,  bringing  the  infinitely  dis- 
tant near  to  the  investigator's  eye,  enlarging  the  infinitely  small, 
and  even  rendering  the  invisible  visible  on  the  astronomer's 
photographic  plate  and  in  the  coloured  sections  of  the  micro- 
scopist,  revealing  to  the  one  the  marvels  of  the  heavens,  and 
to  the  other  the  secrets  of  the  most  diminutive  living  beings. 


It  is  not,  however,  my  intention  now  to  dwell  upon  the 
development  of  the  physical  sciences  -and  their  influence  in 
changing  the  various  circumstances  of  human  life ;  I  purpose 
to  deal  only  with  the  development  of  biology,  which  cannot 
boast  of  such  wide-reaching  triumphs.  Nevertheless,  the 
history  of  biology  in  the  nineteenth  century  forms  part  of  the 
history  of  the  human  intellect,  and  is  an  instructive  piece  of 
what  may  be  called  internal  history,  of  greater  importance 
to  mankind  than  a  merely  superficial  examination  might  lead 
us  to  suppose.- 


We  must  begin  by  clearly  understanding  what  we  mean 

by  biology.     What  is  biology  ?   As  the  name  tells  us,  it  is  the 

science  of  life  and  of  living  creatures.     This  is  biology  in  the 

widest  sense  of  the  word,  and  it  coincides  with  its  oldest 

historical  signification,  as  it  occurs  in  scholastic  philosophy. 

Biology,  or  the  study  of  living  creatures,  is  closely  connected 

with  cosmology,  or  the  study  of  the  bodies  composing  the 

universe,  for,  strictly  speaking,  the  study  of  living  creatures 

includes  the  whole  study  of  plants,  animals  and  men,  but  this 

is  so  vast  a  territory  that  we  generally  apply  the  name  biology 

to  one  comparatively  small  subdivision  of  it,  and  speak  of  the 

biology  of  plants  and  animals  in  contradistinction  to  their 

morphology,  physiology,  and  morphogeny.     Morphology  deals 

with  the  forms   and   component  parts   (organs,  tissues,  and 

cells)  of  organisms.     The  history  of  individual  development, 

or  Morphogeny,  deals  with  the  growth  of  the  organic  forms 

from  the  egg  to  maturity.     Physiology  discusses  the  functions 

of  the  various  parts  of  the  organism,  and  establishes  their 

relations  to  the  process  of  life  and  also  the  chemical  and 

physical  laws  regulating  their  activity.     Finally,    Biology  is 

concerned  with  the  external  activities  affecting  the  organisms 

as  individuals,  and  consequently  governing  their  relation  to 

all  other  organic  beings  as  well  as  to  the  inorganic  world. 

In  this  respect  biology  differs  from  Psychology,  the  proper 

subjects  of  which  are  the  processes  of  sensitive  and  intellectual 

life — essentially  internal  activities,  although  these  frequently 

B  2 


come  within  the  scope  of  biology  in  virtue  of  their  outward 

In  the  narrower  sense  of  the  word,  therefore,  biology  may  be 
defined  as  the  science  dealing  with  the  mode  and  relations  of 
life  in  animals  and  plants.  Human  biology  forms  a  distinct 
branch  of  knowledge,  forming  a  part  of  anthropology,  and  is 
no  longer  regarded  as  belonging  to  biology  in  the  more  restricted 
sense  of  the  word,  now  generally  accepted  by  scientific  writers. 

With  regard  to  the  meaning  of  the  word  '  biology '  and  the  most 
convenient  definitions  to  be  assigned  to  it,  there  are  many  different 
opinions,  only  a  few  of  which  can  be  mentioned  here  briefly.  Almost 
all  scientific  men  agree  in  retaining  the  old  name  '  biology '  (in  the 
Avider  sense)  to  denote  the  whole  mass  of  knowledge  regarding 
life  and  living  creatures.  1  But  there  is  great  diversity  of  opinion 
as  to  the  designation  of  the  special  branch  of  that  science,  which 
we  have  called  oiology  in  the  narrower  sense.  German  zoologists 
used  to  call  it  simply  biology,  until  Ernst  Haeckel  suggested  the 
name  (Ecology.  (Ecology  means  '  study  of  dwelling  '  or  '  science 
of  keeping  house/  it  approaches  the  more  restricted  meaning  of 
biology,  but  does  not  cover  it.  This  new  name  has  found  favour 
not  only  with  many  zoologists,  but  also  with  botanists.  Fr. 
Delpino,'^  F.  Ludwig,3  and  J.  Wiesner  *  speak  of  the  phenomena 
of  plant  life  as  the  biology  of  plants,  whereas  other  botanists,  such 
as  K.  v.  Wettstein,5  prefer  the  name  oecology  of  plants. 

Fr.  Dalil  was  the  first  German  zoologist  to  suggest  the  adoption 
of  Ethology,  or  science  of  the  habits  of  life,  a  word  first  introduced 
by  French  scientific  writers  to  replace  biology  in  the  narrower  sense. 6 

This  new  name  would  certainly  be  more  applicable  to  animal 
biology  than  Haeckel's  oecology,  but  it  is  not  applicable  at  all  to 
plants,  as  we  can  speak  of  '  habits  of  life '  only  with  reference  to 
creatures  that  possess  instinct  and  psychological  life.  If  we  are 
to  have  a  new  name,  it  ought  to  be  applicable  both  to  plants  and 
to  animals  with  regard  to  their  phenomena  of  life. 

An  eminent  botanist,  J.  Keinke,7  is  of  opinion  that  we  can 
dispense  with  the  word  *  biology '  in  the  narrower  sense,  and,  in 
order  to  avoid  confusion  when  it  is  used  in  its  wider  sense,  he 
suggests  the  simple  expression  '  Mode  of  life  among  animals  and 

1  Cf.  for  instance,  0.  Hertwig's  Entwicklung  der  Biologic,  im  19  Jahrhundert, 
Jena,  1900. 

2  Pensieri  sulla  Biologia  vegetale,  efcc.,  Nuovo  Cimento,  XXV,    Pisa,  1867. 

3  Lehrbuch  der  Biologic  der  Pflanzen,  Stuttgart,  1895. 

4  Biologic  der  Pflanzen,  1902,  I. 

5  Leitfaden  der  Botanik  fur  die  oberen  Klassen  der  Mittelschulen,  1901,  1. 

6  Cf.  Wasmann,  '  Biologic  oder  Ethologie  ? '  (Biolog.  Zentralblatt,  XXI,  1901, 
No.  12,  pp.  391-400). 

1  'Was  heisst  Biologic  ? '  (Natur  und  Schule,  I,  1902,  part  8,  p.  449,  &c.). 


plants '  as  a  substitute  for  the  word  in  its  more  restricted  significa- 
tion. This  designation  is  clear  and  convenient  enough,  but  I 
scarcely  think  that  it  fulfils  the  requirements  of  science,  for  we  need 
some  internationally  intelligible  word  for  '  mode  of  life  '  or  '  Lebens- 
weise/  formed  from  Greek  roots  on  the  analogy  of  '  Morphology/ 
'  Physiology/  &c. 

To  supply  this  deficiency  the  word  bionomy  or  bionomics  has  been 
introduced  in  England  1  and  North  America,^  and  this  is  perhaps 
the  best  word  yet  suggested  to  designate  the  mode  of  life  of  animals 
and  plants,  for  it  denotes  the  laws  governing  life  '  (/^os-vojuos), 
and  so  means  exactly  what  we  defined  as  biology  in  the  narrower 
sense,  and  at  the  same  time  it  avoids  the  ambiguity  of  the  word 
biology.  I  should  have  no  objection  to  accept  this  new  name 
Bionomics,  to  designate  the  mode  of  life  among  animals  and  plants ; 
but  as  it  is  not  yet  current  in  Germany,  I  may  be  permitted  to 
retain  the  old  name. 

The  experimental  study  of  the  laws  of  heredity  and  variation  has 
recently  been  called  Biometry. .3  In  1901  a  new  periodical  appeared 
in  Cambridge  (England)  entitled  Biometrica :  A  Journal  for  the 
Statistical  Study  of  Biological  Problems.  Biometry  is,  therefore, 
synonymous  with  Statistical  Biology. 

The  following  simile  may  serve  to  illustrate  more  clearly 
the  original  meaning  of  the  word  biology,  and  the  various 
modifications  which  it  has  undergone  owing  to  the  progress 
made  by  science  in  the  nineteenth  century. 

Biology,  in  its  widest  signification,  embraces  all  that  we 
know  about  living  creatures,  and  we  may  compare  it  with  a 
lofty  tree  having  three  main  boughs,  but  many  branches,  and 
its  stem,  boughs,  and  branches  are  the  biological  sciences.  The 
tree  is  crowned  by  twigs  shooting  from  the  main  trunk,  and 
this  crown  represents  the  science  dealing  with  man,  or  anthro- 
pology, and  the  topmost  of  its  twigs,  rising  up  into  the  domain 
of  the  intellectual  sciences,  is  the  psychology  of  man  and 
nations.  Below  it  is  human  biology  in  the  narrower  sense, 
then  human  physiology,  human  morphology  and  the  history 
of  human  development,  all  having  many  subordinate  twigs, 

1  Cf.,  e.g.,  G.  K.  Marshall  and  E.  B.  Poulton,  '  Five  Years'  Observations 
and  Experiments  on  the  Bionomics  of  South  African  Insects '  (Transactions  of 
the  Entomological  Society,  London,  1902,  part  3). 

2  Cf.  Ch.  S.  Minot,     'The   Problem   of   Consciousness   in    its    Biological 
Aspects'   (Proceedings  of   the    American  Association  for  the   Advancement  of 
Science,  XXXI,  p.  272). 

3  Cf .  Chr.  Schroder,  'Bine  Sammlung  von  Ref  eraten  iiber  neuere  biometrische 
Arbeiten'   (A  Ilgemeine,  Zeitschrift  fur  Entomologie,  IX,  1904,  Nos.  11  and  12, 
]>  228,  &c.). 


bearing,  for  the  most  part,  the  same  names  as  the  correspond- 
ing ramifications  of  the  zoological  stem.  Some  few  branches 
belonging  to  the  crown  have  names  of  their  own,  to  which 
zoology  supplies  analogies  only ;  such  are  ethnology  and 
archeology,  psychopathology,  and  medicine. 

Below  the  crown  a  great  bough  springs  from  the  main 
trunk  of  the  biological  sciences  :  this  is  zoology.  Its  chief 
offshoots  are  animal  psychology  and  animal  biology  (animal 
bionomics)  and  the  physiology,  morphology,  and  morphogeny 
of  animals.  In  the  course  of  the  nineteenth  century  a  great 
number  of  little  twigs  grew  out  of  each  of  these  branches,  of 
which  only  a  few  can  be  mentioned  here.  Out  of  animal 
biology  or  bionomics  sprang  trophology,  or  the  science  dealing 
with  the  food  of  animals  ;  oecology,  or  the  science  dealing  with 
their  habitations ;  animal  geography,  dealing  with  their 
distribution  ;  and,  further,  their  parasites  have  been  studied, 
and  the  tendency  of  certain  animals  to  live  with  other  animals 
or  near  to  some  particular  plants  (symbiosis).  This  has  given 
rise  to  investigations  of  a  biological  nature  into  the  way  of  life 
of  ants  and  termites,  and  one  of  the  most  fertile  offshoots  of 
modern  biology  is  the  study  of  the  inquilines  among  ants  and 
termites.  We  cannot  do  more  than  name  nervous  physiology 
which,  with  its  offshoots,  cerebral  physiology,  physiology  of 
the  external  organs  of  sense  and  of  the  nerve  tracks,  threatens 
to  take  the  place  of  animal  psychology,  now  said  to  be  out 
of  date.1 

Modern  morphology  has  even  more  ramifications,  branch- 
ing out  in  one  direction  into  systematics,  or  the  science  of 
systematic  classification,  and  in  the  other  into  morphology 
proper,  which  latter  is  subdivided  into  exterior  and  interior 
morphology,  the  interior  comprising  topographical  anatomy, 
histology  or  study  of  the  tissues,  and  cytology  or  study  of 
the  cells — all  three  well-developed  offshoots  of  morphology. 
Moreover,  all  these  branches  of  morphology  have  their  counter- 
parts on  the  physiological  side,  in  the  physiology  of  the  organs, 
tissues,  and  cells. 

Morphogeny,  or  the  history  of  the  development  of  animals, 

1  On  this  subject  cf.  my  article  '  Nervenphysiologie  und  Tierpsychologic  ' 
(Biolog.  Zentralblatt,  XXI,  1901,  No.  1,  pp.  23-32)  and  also  Instinkt  und 
Intelligenz  im  Tierreich,  1905,  chap.  ii. 


has  two  great  branches,  viz.  ontogeny,  or  the  history  of 
individual  growth,  and  phylogeny,  or  the  history  of  the  race 
development.  Ontogeny  is  divided  into  embryology  and  post- 
embryonic  development,  which  includes  the  phenomena  of 
metamorphosis,  metagenesis,  &c.  Finally  we  must  allude  to 
animal  pathology  as  a  branch  of  zoology.  Reference  has 
already  been  made  to  animal  geography  as  a  branch  of  animal 

Nearer  the  root  of  the  tree  springs  the  lowest  bough  of 
biology,  viz.  botany.  Nothing  is  found  on  it  corresponding 
to  the  most  dignified  offshoot  of  the  zoological  bough — animal 
psychology,  because  plants  have  no  consciousness,  and  even 
the  most  sensitive  of  them  show  only  a  faint  resemblance  to 
conscious  life.1 

There  are,  however,  on  the  botanical  bough  a  good  many  off- 
shoots corresponding  to  the  other  parts  of  zoology ;  we  have  the 
biology  (bionomics)  of  plants,  which  includes  plant-geography, 
and  we  have  also  plant-physiology  and  morphology,  plant- 
anatomy  and  cytology,  and  finally  phytopathology.3  The 
botanical  branch  is  further  distinguished  by  possessing  one 
suspiciously  luxuriant  and  poisonous  looking  offshoot,  which 
boldly  rises  up  to  the  branch  of  the  crown  that  we  have  called 
'  medicine,'  and  this  is  bacteriology.  Fortunately  it  has  a 
less  poisonous  side  in  the  phenomena  of  fermentation  and 
assimilation  of  nitrogen,  which  are  in  many  respects  beneficial 
to  man. 

To  our  astonishment  we  see  that  our  tree  bears  one  or  two 
apparently  dead  branches  of  considerable  size  ;  they  spring 
from  the  same  point  of  the  main  trunk  as  the  zoological  and 
botanical  boughs  respectively,  and  they  are  called  palceozoology 
and  palceophytology.  They  are,  however,  by  no  means  really 
dead,  although  they  deal  with  the  extinct  ancestors  of  the 
animal  and  vegetable  kingdoms  of  the  present  day. 

In  the  main  trunk  supporting  the  crown  and  the  branches 

1  Many  modem  botanists  regard  this  analogy  as  constituting  real  identity 
(homology),  but  they  are  certainly  mistaken.  Cf.  for  instance,  Haberlandt, 
Die  Sinnesorgane  im  Pflanzenreich  zur  Perzeption  mechanischer  Reize,  Leipzig, 
1900.  For  a  criticism  on  these  views,  see  J.  Reinke,  Philosophic  der  Botanik, 
1905,  66,  &c.,  83,  &c. 

'z  The  distinction  between  anatomy  and  histology  is  less  marked  in  the 
case  of  plants,  as  their  tissues  do  not  differentiate  themselves  so  sharply 
into  organs  as  do  those  of  animals. 


of  the  tree  of  biological  knowledge  with  all  their  offshoots  and 
twigs  rises  a  stream  of  sap,  representing  the  comparative  and 
generalising  elements  belonging  to  all  the  biological  sciences  ; 
these  connect  all  the  parts  of  the  tree  with  one  another  and 
enable  us  to  view  them  intelligently  as  a  whole,  and  at  the 
same  time  they  enlighten  us  as  to  its  growth.  Comparative 
psychology  effects  a  close  connexion  between  the  zoological 
branch  and  the  crown  of  the  tree  ;  comparative  biology  and 
physiology,  comparative  morphology,  anatomy  and  histology, 
comparative  cytology  and  comparative  morphogeny  send 
streams  of  life  through  all  the  branches  and  twigs  of  the  great 
tree,  and  show  that  they  are  all  living  parts  of  one  vast  whole. 
Chemistry  and  physics,  too,  and  especially  mechanics  of 
organic  structures,  are  represented  in  the  roots  of  the  tree,  as 
biochemistry  and  biophysics,  and  they  connect  it  with  the 
surrounding  domain  of  the  inorganic  sciences.  But  the 
quintessence  of  all  the  sap  flowing  in  the  tree  of  biological 
knowledge  is  the  scientific  conception  of  life,  and  the  trunk  of 
the  tree,  which  supports  and  nourishes  all  these  branches  and 
twigs,  is  the  science  of  life. 


We  have  just  seen  how  the  tree  of  biological  sciences  grew 
rapidly  in  the  nineteenth  century,  and  produced  an  indescrib- 
able abundance  of  offshoots,  leaves,  blossoms  and  fruit  on 
branches  previously  bare.  Let  us  now  consider  the  origin  of 
this  tree  and  how  it  fared  whilst  still  an  insignificant  seedling. 

It  was  not  planted  first  in  the  year  1800,  nor  did  it  suddenly 
develop  on  New  Year's  Day,  1801,  into  a  trunk  sturdy  enough 
to  support  all  the  branches  and  twigs  which  the  new  century 
was  destined  to  add  to  it.  It  is  far  older  than  this,  and  we 
can  trace  its  history  for  several  thousand  years.  The  seed, 
whence  this  tree  has  grown,  was  planted  when  God  breathed 
into  the  first  man  the  breath  of  life,  as  we  read  in  the  beautiful 
figurative  language  of  Holy  Scripture.  The  breath  of  God's 
spirit,  dwelling  in  man,  its  all-embracing  power  of  understanding 
and  its  never  satisfied  thirst  for  knowledge,  form  the  hidden 
motive  power,  the  inner  living  force  of  this  tree.  Man  has 
always  been  possessed  by  a  thirst  for  knowledge,  both  among 


civilised  nations  and  among  the  wild  children  of  nature.  The 
Eskimo  of  the  present  day  adorns  the  walrus  ivory  implements 
used  in  shooting  his  arrows  with  dogs'  heads  and  outlines  of 
reindeer,  birds  and  human  beings,  showing  that  the  shapes  of 
the  living  creatures  around  him  have  deeply  impressed  them- 
selves upon  his  mind ;  and,  in  the  same  way,  the  cave-dwellers 
of  Central  Europe  scratched  rough  sketches  of  fish,  horses  and 
other  animals  on  reindeer  bones.  Even  if  the  famous  repre- 
sentation of  a  long-haired  mammoth  with  a  long  mane,  which 
was  found  on  a  piece  of  a  mammoth's  tooth,  proves  not  to  be 
genuine,  and  the  much  finer  engraving,  on  a  reindeer  antler 
from  the  cavern  at  Kessler,  of  a  reindeer  grazing,  is  in  all 
probability  a  modern  forgery,  still,  as  J.  Ranke  says,1  it  is 
difficult  to  say  exactly  when  the  germ  of  biological  research 
latent  in  the  mind  of  man  first  assumed  a  scientific  form,  and 
appeared  as  a  young  plant  above  the  ground.  We  know, 
however,  one  famous  gardener,  who  tended  the  little  tree 
most  skilfully,  and  that  is  Aristotle  the  Stagirite. 

Aristotle  had  predecessors,  no  doubt ;  the  animal  system 
devised  by  the  followers  of  Hippocrates  of  Cos  had  already 
prepared  the  way  for  him,2  yet  he  certainly  deserves  to  be 
called  the  Father  of  Biological  Science.  His  classical  works 
'  Historia  animalium,'  *  De  partibus  animalium,'  and  *  De 
generatione  animalium '  are  the  foundations  of  our  scientific 
systematic  classification  and  biology,  of  morphology,  anatomy, 
and  morphogeny.3  In  his  writings  he  actually  mentions  500 
kinds  of  animals.4  As  he  does  not  allude  to  many  other 
varieties  that  are  very  common  and  occurred  in  ancient  Greece 
in  his  day,  we  must  assume  that  he  did  not  think  it  necessary 
to  speak  of  all  the  animals  with  which  he  was  familiar.  He 
divides  animals  into  two  chief  classes,  Zvaifta  or  with  blood 
(more  correctly  red-blooded),  and  avaifia  or  bloodless,  and 

1  Der  Mensch,  II,. Leipzig  and  Vienna,  1894,  459,  &c. 

2  Cf .  R.  Burckhardt, '  Das  koische  Tiersystem,  eine  Vorstuf e  der  Zoologischen 
Systematik  der  Aristoteles '  (reprinted  from  the  Verhandl.  der  naturf.  Gesell- 
schaft  in  Basel,  XV,  1902,  part  3,  pp.  377-414). 

3  R.  Burckhardt,  '  Das  erste  Buch  der  aristotelischen  Tiergeschichte '  (Zoo- 
logische  Annalen,  I,  Wiirzburg,  1904,  part  1).     Also  '  Zur  Geschichte  der  biolo- 
gischen  Systematik '  ( VerJiandlungen  der  Naturf.  Gesellschajt  in    Basel,  XVI, 
1903,  388-440). 

4  We  cannot  here  discuss  their  division  into  different  classes.     Gunther 
remarks  that  the  number  of  varieties  of  fish  known  to  Aristotle  seems  to 
have  been  115  (Handbuch  der  Ichthyologie,  1886,  p.  3). 


this  division  practically  answers  to  the  modern  classification 
into  vertebrates  and  invertebrates.  The  eight  yevrj  /^ejccrra, 
or  chief  classes  of  the  Aristotelian  system,  agree  roughly  with 
our  chief  classes  in  the  animal  kingdom.  The  conception  of 
the  eZSo?  or  species,  introduced  by  Aristotle,  underlies  our 
modern  conception  of  it.  But  the  great  philosopher  was  not 
only  a  pioneer  in  systematic  classification,  he  was  equally 
eminent  as  a  morphologist,  an  anatomist,  a  biologist,  and  an 
embryologist.  He  compared  animals  with  regard  to  their 
form  and  structure,  and  studied  their  mode  of  life  and  the 
history  of  their  development. 

How  great  a  biologist  Aristotle  was  is  proved  by  the  fact 
that  some  of  his  discoveries  were  rediscovered  in  the  nineteenth 
century,  and  were  regarded  as  brand-new  triumphs  of  modern 
science.  Aristotle  knew  that  many  sharks  do  not  only  produce 
their  young  alive,  but  that  in  their  case  the  young  before  their 
birth  are  nourished  by  a  process  closely  resembling  that  of 
mammals  (development  of  a  placenta).  This  fact  was  re- 
discovered by  Johannes  Miiller,  a  famous  anatomist  and 
zoologist  (1801-58).  Moreover,  Aristotle  was  aware  of  the 
difference  between  male  and  female  cephalopods,  and  had 
observed  that  young  cuttlefish  possess  a  vitelline  sac  near  the 
mouth.  The  accuracy  of  these  old  observations  has  been 
completely  proved  by  modern  research.  Bretzl  has  thrown  an 
astonishing  light  upon  the  extent  and  importance  of  the 
botanical  knowledge  possessed  by  Greeks  of  Aristotle's  time.1 

When  we  consider  the  well-merited  prestige  enjoyed  by 
Aristotle  as  founder  of  biology,  when  we  remember  the  enor- 
mous wealth  of  knowledge,  interspersed  though  it  be  with  many 
errors,  contained  in  his  works,  we  cease  to  wonder  that  for  two 
thousand  years  everyone,  who  studied  biology  at  all,  studied 
Aristotle  almost  exclusively,  quoted  Aristotle,  made  extracts 
from  Aristotle,  and  wrote  commentaries  on  Aristotle.  The 
work  of  the  Younger  Pliny  in  this  department  is  insignificant 
in  comparison  with  that  of  his  great  predecessor,  and  even 
in  some  respects  shows  a  falling  off.  Pliny,  however,  has  been 
the  chief  source  of  information  for  most  of  the  students  of 
nature  both  of  antiquity  and  of  the  Middle  Ages,  who  derived 

1  Die   bolanischen   Forschungen   des    Alexanderzuges,    Leipzig,    1903.     Cf. 
the  review  in  the  Botanisches  Zentralblatt,  XCIII,  1903,  p.  97,  &c. 


from  him  their  biological  knowledge,  and  adopted  as  genuine 
all  the  stories  found  in  Pliny's  *  History  of  Animals,'  without 
in  any  way  testing  their  truth.  A  standard  work  of  this 
description  is  the  famous  '  Physiologus  '  or  '  Bestiarium,'  in 
which  all  the  legends  connected  with  zoology  are  collected, 
with  edifying  morals  appended  to  them. 

It  would  he  unfair  not  to  acknowledge  that,  among  the 
great  scholastic  philosophers  of  the  thirteenth  century,  there 
were  a  number  of  men  who  did  their  best  to  carry  on  inde- 
pendent scientific  research.  Besides  St.  Thomas  Aquinas, 
the  Dominican  Order  produced  in  that  century  three  great 
men,  conspicuous  not  so  much  for  their  scholasticism,  as  for 
their  proficiency  in  another  department  of  knowledge. 

These  were  Thomas  of  Chantimpre,  Vincent  of  Beauvais,  and 
Albertus  Magnus  or  Albert  the  Great  (1193-1280),1  of  whose 
treatise  upon  animals  Victor  Carus  says,  in  his  '  Geschichte  der 
Zoologie,'  p.  226,  that,  in  comparison  with  the  works  of  the 
two  previously  mentioned  writers,  it  is  far  more  thorough 
and  composed  with  greater  self-confidence. 

Thomas  of  Chantimpre  was  a  pupil  of  Albertus  Magnus,3 
and  that  Vincent  of  Beauvais  used  his  books  is  proved  by 
his  numerous  quotations  from  them.  Although,  like  all  his 
predecessors,  Albert  the  Great  based  his  work  on  Aristotle, 

1  Cf.  F.  A.  Pouchet,  Histoire  des  Sciences  naturelles  au  moyen-age,  ou  Albert 
le  Grand  et  son  epoque  consider  cs  comme  point  de  depart  de  Vecole  experimentale, 
Paris,  1853.      Cf.  also  Fr.  Ehrle,  S.J.,    '  Der  selige  Albert  der  Grosse,'    in 
Stimmen  aus  Maria-Laach,   XIX,  1880  ;    G.   v.   Hertling,  Albertus  Magnus, 
Beitrdge  zu  seiner  Wilrdigung,  written  in  honour  of  the  600th  anniversary  of 
his  death,  Cologne,  1880;    E.  Michael,   S.J.,  Geschichte  des  deutschen  Volkes 
vom  13  Jahrhundert  bis  zum  Ausgang  des  Mittelalters,  III,  1903,  pp.  445-460 ; 
Arthur   Schneider,   Die   Psychologie   Albert  des   Grossen :   Nach  den   Quellen 
dargestellt,  I,  1903,  Vorwort  VIII. 

2  He  describes  himself  as  an  auditor  eius  per  multum  tempus.     (Thomas 
Cantipratanus,  Bonum  universale,  Duaci,  1627,  1.  2,  c.  57,  §  50,  p.  576.     Cf. 
E.  Michael,  S.J.,  '  Albert  der  Grosse,'  in  the  Zeitschrift  fur  Katholische  Theo- 
logie,  1901,  part  1,  p.  43.)     Borman  is  therefore  probably  mistaken  in  thinking 
that  Thomas  of  Chantimpre's  work  was  one  of  Albert  the  Great's  chief  sources 
of  information  in  the  compilation  of  his  book  on  animals.     V.   Carus  falls 
into  the  same  mistake  in  his  Geschichte  der  Zoologie,  p.  227.     Cf.  also  Alex. 
Kaufmann,  Thomas  von  Chantimpre,  Cologne,  1899.     Thomas  was  a  canon 
regular  in  the  Augustinian  monastery  at  Chantimpre  before  he  entered  the 
Dominican  Order  in   1232.     His  book,  entitled  Liber  de  rerum  natura,  was 
subsequently  translated  into  German  by  Konrad  Megenberg,  who  belonged 
to  the  cathedral  chapter  at  Ratisbon.     Its  German  title  is  Buch  der  Natur 
(Book  of  Nature),  and  it  records  the  results  of  much  independent  research. 
The   same   author's  work  on  bees  (Bonum  universale  de  apibus]  is  a  pious 
picture  of  manners  rather  than  a  treatise  on  natural  history. 


he  took  more  pains  than  any  of  them  to  make  independent 
observations  of  his  own.  His  treatise  on  animals  consists  of 
twenty-six  books,  of  which  nineteen  correspond  to  the  writings 
of  Aristotle,  whilst  seven  are  of  independent  origin.1 

Book  XX,  the  first  of  those  containing  his  own  results, 
deals  with  the  nature  of  animals'  bodies  in  general,  and  Book 
XXI  with  the  degrees  of  perfection  attained  by  them 
(de  gradibus  perfectorum  et  imperfectorum  animalium),  a  quite 
modern  idea  in  classification,  on  the  lines  of  comparative 
morphology  of  animals.  The  remaining  five  books  deal  with 
animals  singly,  arranged  alphabetically  within  the  larger 
groups.  These  seven  books  show  conclusively  that  the  author 
was  not  content  to  write  a  commentary  on  Aristotle,  but 
aimed  at  rendering  his  work  more  complete  by  adding  the 
results  of  his  own  investigations. 

Albert  the  Great's  seven  books  *  De  vegetabilibus  et  plantis,' 
which  contain  his  views  on  botany,  have  been  carefully  studied 
and  justly  appreciated  by  E.  Meyer,  in  his  *  Geschichte  der 
Botanik,'  IV,  Konigsberg,  1857,  but  the  more  important 
work  on  zoology  has  hitherto  met  with  far  too  slight  recog- 
nition among  scientific  men.  An  attempt  to  display  its 
merits,  made  by  Karl  Jessen  in  1867,  was  frustrated,  owing 
to  the  defective  state  of  most  editions  of  Albert  the  Great's 

E.  von  Martens  subsequently  published  some  observations 
on  several  of  the  mammals  mentioned  by  him,  and  Victor  Carus 
has  devoted  a  few  pages  to  Albert  the  Great  in  his  '  Geschichte 
der  Zoologie,'  but  without  discussing  his  work  in  detail.3 
Although  Carus  is  by  no  means  a  partisan  of  the  Church,  he 
feels  bound  to  confess,  on  p.  224,  that  '  Albert,  to  whom  the 
cognomen  "  Great  "  may  justly  be  conceded,  is  undoubtedly 
the  chief  writer  of  the  thirteenth  century  on  the  subject  of 
natural  science.'  If  Carus  had  adhered  to  the  principle  which 
he  himself  laid  down,  and  had  foreborne  to  judge  Albert  the 
Great  as  a  zoologist  by  the  standard  of  a  modern  writer  on 

1  In  the  complete  edition  of  Albert  the  Great's  works,  published  in  Paris 
by  Vives,  the  treatise  on  animals  is  contained  in   vol.  xi  (De  animalibus  pars 
prior)  and  vol.  xii  (De  animalibus  pars  altera). 

2  '  Alberti    magni    historia    animalium '      (Archiv    fur    Naturgeschichte, 
XXXIII,  vol.  i,  1867,  pp.  95-105). 

3  Munich,  1872,  pp.  224-237. 


science,  he  would  probably  have  spoken  in  more  favourable 
terms  of  his  achievements  in  zoology. 

Although  Albert  the  Great  could  not  completely  disentangle 
himself  as  a  zoologist  from  the  prejudices  and  fancies  of  his 
predecessors,  his  merit  lies,  not  merely  in  his  having  gone 
back  from  Pliny  to  Aristotle,  but  also  in  his  having  led  the 
way  to  independent  research,  which  does  not  rely  blindly 
upon  authority,  but  looks  for  itself.1 

R.  Hertwig  is  perfectly  correct  in  stating  in  the  most 
recent  edition  (seventh)  of  his  *  Lehrbuch  der  Zoologie '  (1905, 
p.  7)  that  Albert  the  Great  even  began  to  collect  his  own 
zoological  observations.  In  many  passages  of  his  work  on 
animals  he  refers  to  his  own  investigations,  and,  when  he 
describes  anything,  he  frequently  adds  a  remark  to  the  effect 
that  he  has  himself  seen  the  thing  in  question,  and  even  possesses 
it  in  his  collection.  He  devotes  several  chapters  to  the  habits 
of  falcons,  which  he  seems  to  have  studied  with  particular 
interest.  In  one  place  he  tells  us  that  he  took  a  short  sea 
voyage  for  zoological  purposes,  and  on  the  shore  of  an  island 
he  collected  ten  or  eleven  kinds  of  *  bloodless  sea-beasts.' 
After  recording  the  various  tales  told  about  the  propagation 
of  fish,  he  adds  :  '  I  believe  that  none  of  all  this  is  true,  for 
I  have  myself  made  diligent  investigations,  and  have  questioned 
the  oldest  fishermen  engaged  in  salt  and  fresh  water  fishing,' 
and  he  proceeds  to  give  the  results  of  his  observations  and 
inquiries.  He  declares  that  by  personal  observation  he  has 
disproved  the  popular  theory  that  the  left  legs  of  a  badger 

1  Men  such  as  Albert  the  Great  are  enough  to  refute  the  discovery  made 
by  certain  followers  of  Darwin,  that  Christianity  has  '  stifled  the  spirit  of 
scientific  research  '  and  has  '  caused  a  kind  of  hostility  to  the  idea  of  busying 
the  mind  with  natural  objects.'  It  is  unfortunate  that  such  prejudiced 
statements  have  found  their  way  into  even  our  modern  text-books  of  zoology. 
See,  for  instance,  R.  Hertwig,  Lehrbuch  der  Zoologie,  1900,  p.  7.  The  following 
words,  which  I  quote  from  Hertwig,  cannot  be  applicable  to  Albert  the  Great : 
'  The  question  how  many  teeth  a  horse  has  was  discussed  in  many  contro- 
versial treatises,  in  which  the  authors  used  all  the  heavy  artillery  at  their 
disposal,  but  it  did  not  occur  to  one  of  the  learned  men  to  look  inside  a  horse's 
mouth  and  see  for  himself.'  It  is  to  the  credit  of  the  author  of  the  above- 
mentioned  excellent  text-book  of  zoology,  that  the  words  just  quoted  have 
been  omitted  in  the  two  last  editions  of  his  book  (1903  and  1905).  It  is 
satisfactory  to  observe  that  the  achievements  of  mediaeval  scholars  in  the 
domain  of  natural  science  are  gradually  receiving  fairer  treatment,  and  are 
being  judged  by  a  more  unprejudiced  standard.  Cf.  also  J.  Norrenberg, 
*  Der  naturwissenschaftliche  Unterricht  in  den  Klosterschulen '  (Scientific 
Instruction  in  Monastic  Schools),  in  Natur  und  Schule,  III,  1904,  part  4, 
pp.  161-169. 


are  shorter  than  the  right  legs,  -and  he  relegates  the  stories  of 
geese  growing  on  trees,  and  other  zoological  marvels,  into  their 
proper  sphere  as  fictions  of  the  imagination.1  It  is  true  that 
his  statements  are  interspersed  with  a  good  many  mistakes. 
He  is  right  in  saying  that  flies  have  two  wings,  but  wrong  in 
giving  them  eight  legs — and  his  famous  pupil,  Thomas  Aquinas, 
is  falsely  accused  of  having  reckoned  ants  among  the  reptilia 
quadrupedia,  and  thus  of  having  fallen  into  an  opposite  error.3 
It  is  hardly  necessary  to  point  out  how  impossible  it  was  for 
him  to  correct  the  old  legends  with  reference  to  exotic  animals, 
and  so  he  says  that  the  porcupine  shoots  its  quills  at  its  enemies, 
that  the  wild  unicorn  grows  tame  when  caressed  by  a  maiden, 
&c.  We  ought  to  bear  in  mind  that  to  a  German  student 
of  nature  in  the  thirteenth  century  no  other  source  of  informa- 
tion about  foreign  animals  was  accessible  than  the  old  fabulous 
stories.  What  pains  Albert  the  Great  took  to  obtain  trust- 
worthy information  about  animals  that  he  had  never  seen, 
is  proved  by  his  admirable  account  of  the  methods  then  in  use 
in  the  whalefishery. 

Careful  studies  in  another  quarter  have  recently  shown 
that  Albert  the  Great  followed  an  independent  method  of 
investigation.  Dr.  E.  Hertwig,  Professor  of  Zoology  at  the 
University  of  Munich,  suggested  to  Dr.  H.  Stadler  to  make  a 
critical  examination  of  Albert's  zoology  and  botany.  The 
full  result  of  this  examination  has  just  been  published  in  the 
Forschungen  zur  Geschiclite  Baierns,  XIV,  1906,  first  and  second 
parts,  pp.  95-114,  but  Stadler  communicated  a  good  deal  of 
it  previously,  at  a  lecture  delivered  on  March  20,  1905,  to  the 
*  Verein  fur  Naturkunde  '  in  Munich.  The  title  of  the  lecture 
was  :  '  Albert  the  Great  as  an  independent  student '  ;  I 
•subjoin  some  extracts  from  it  : — 

This  very  prolific  writer  was  a  scholastic,  but  lie  occupies  a 
position  on  a  level  with  Aristotle  rather  than  subordinate  to  him, 

1  The  story  of  the  geese  growing  on  trees  probably  originated  in  the  fact 
that  the  barnacle  goose   (Lepas  anatifera)  often  attaches  itself  to  floating 
tree  trunks. 

2  In  the  Summa  Theologiae,  I,  q.  72,  ad  2.      In  Vives'  edition  (1871)  the 
passage  reads  as  follows  :    '  Per  reptilia  vero  (intelleguntur)  animalia,  quae 
vel  non  habent  pedes  .  .  .  vel  habent  breves,   quibus  parum  elevantur  ut 
lacertae  et  tortucae.'     There  is  a  note  on  the  word  tortucae  :    '  Sic  codices, 
Bed  nescio  qua  incuria  in  Parmensi  et  in  omnibus  editionibus  formicae.'    Tortuca 
is    tartaruga,    tortue,   tortoise,  and  is  rightly  reckoned    among    the  reptiles, 
only  a  constantly  repeated  misprint  has  turned  tortoises  into  ants  ! 


and  did  not  simply  reproduce  Aristotle's  statements,  but,  as  far 
as  he  could,  explained,  completed  and  expanded  them.  He  dis- 
played great  shrewdness  and  keen  intelligence  in  carrying  on  his 
favourite  observations  on  the  animals  and  plants  of  Germany, 
whence  he  derived  the  evidence  for  his  scientific  statements  that 
he  based  upon  Aristotle.  His  writings  therefore  contain  all  the 
information  on  natural  history  possessed  by  the  people  of  Germany 
in  his  day  ;  he  describes  the  life  of  animals  as  observed  by  intelligent 
huntsmen  and  farmers,  fishermen  and  bird-catchers  ;  everywhere  the 
biological  element  and  his  own  personality  are  prominent,  and 
for  this  reason  his  writings  form  a  sharp  contrast  to  the  dry 
book-learning  of  the  periods  preceding  and  following  his  lifetime. 
It  is  true  that  in  dealing  with  botany  he  follows  the  lines  of  the 
pseudo- Aristotelian  work  '  De  plantis/  really  written  by  Nicholas 
Damascenus,  but  under  the  form  of  excursus  he  gives  a  far  better 
account  of  the  subject,  based  upon  his  own  observations.  He 
describes  very  correctly  the  vascular  bundles  of  the  plantain  leaf  and 
the  medullary  rays  of  the  vine,  and  divides  plants  into  two  classes, 
cortical  and  tunical,  a  division  approximately  corresponding  to 
that  of  monocotyledonous  and  dicotyledonous.  He  distinguishes 
parenchyma  and  bast-fibres  in  the  large  stinging  nettle,  hemp 
and  flax  ;  he  knows  the  difference  between  the  inner  and  outer 
bark,  and  the  importance  of  each  to  the  life  of  a  plant.  He  has 
observed  the  square  stem  of  the  deadnettle,  and  the  diversity  in 
growth  between  plants  in  isolation  and  when  cramped  for  space. 
He  describes  very  clearly  the  difference  between  a  thorn  and  a 
sting ;  he  attempts  a  classification  of  leaves  according  to  the 
shape,  notices  that  plants  with  woody  stems  have  bud-scales, 
and  herbaceous  plants  have  naked  buds,  and  he  recognises,  as  a 
peculiarity  of  the  grape  vine,  the  fact  that  fruit  and  tendrils  are 
opposite  to  the  foliage  leaves. 

In  speaking  of  blossoms  he  draws  attention  to  their  various 
forms  of  insertion,  and  mentions  stamens,  pistil  and  pollen,  although 
he  confuses  the  pollen  with  wax.  He  comments  upon  the  deciduous 
calyx  of  the  poppy,  tries  in  a  very  primitive  fashion  to  classify 
the  forms  of  the  corolla,  insists  upon  the  importance  of  the  seed 
in  preserving  the  species,  and  gives  a  very  fair  classification  of 
fruits.  The  position  and  the  significance  of  the  ovules  and  of 
the  tissues  connected  with  nutrition  did  not  escape  his  notice. 
The  sixth  book,  *  De  vegetabilibus/  contains  many  admirable 
descriptions  of  single  plants,  especially  of  the  mistletoe,  the  hazel, 
the  alder,  the  ash,  the  date-palm,  the  poppy,  borage  and  rose,  and 
in  the  case  of  the  last-mentioned  he  gives  an  excellent  account 
of  the  aestivation  of  the  calyx  and  of  the  alternation  of  the  parts 
of  the  flower,  and  suggests  the  true  explanation  of  their  significance. 

We  may  speak  in  similar  terms  of  his  work  on  zoology,  for 
which,  however,  we  are  unfortunately  obliged  to  use  the  very 
unsatisfactory  edition  published  by  Auguste  Borgnet  in  Paris,  1891, 


so  that  much  in  it  appears  open  to  question.  Of  animals  known 
in  Germany,  Albert  begins  by  describing  the  German  marmot  and 
the  earless  marmot,  the  two  kinds  of  marten,  the  garden  dormouse 
and  the  common  dormouse,  and  he  is  the  first  writer  who  alludes 
to  the  chamois,  the  badger,  the  rat,  the  ermine  and  the  polecat.! 

He  gives  charming  accounts  of  the  mole,  the  marmot  and  the 
squirrel ;  he  knows  the  Lepus  variabilis  of  the  North  and  the  polar 
bear  ;  he  describes  a  whaling  expedition  and  remarks  that  in  his 
day  the  elk,  the  bison,  and  the  aurochs  were  to  be  found  only  in 
the  extreme  east  of  Germany.  His  description  of  the  cat  displays 
great  sympathy  with  animals  and  very  sharp  powers  of  observation. 

In  dealing  with  birds,  he  discusses  the  various  falcons  in  the 
greatest  detail,  but  he  is  well  acquainted  with  the  other  birds  of 
prey.  He  speaks  of  the  peculiar  structure  and  purpose  of  the 
woodpecker's  claws,  and  considers  the  distribution  of  the  hooded 
crow  and  the  habits  of  migratory  birds. 

Blackcock,  grouse,  and  heathcock  were  familiar  to  him,  and 
he  knew  many  kinds  of  singing  birds  (four  varieties  of  finches, 
two  of  sparrows  and  three  of  swallows),  also  the  nutcracker  and 
kingfisher ;  he  describes  the  nest  of  the  magpie  and  the  habits  of 
the  cuckoo  with  great  accuracy.  The  lecturer  proposed  to  speak 
of  Albert  the  Great's  knowledge  of  fishes  on  another  occasion  ; 
he  stated  that  Albert  had  dissected  insects  and  had  perhaps  recog- 
nised the  digestive  system  and  heart.  He  gives  a  correct  account 
of  the  development  of  cockchafers  and  wasps,  and  also  of  caterpillars 
and  their  spinning  process,  and  of  the  habits  of  the  ant-lion.  Of 
other  creatures,  the  best  description  given  as  the  result  of  his  own 
observation  is  perhaps  that  of  the  jelly-fish. 

Among  the  learned  Franciscans  of  the  thirteenth  century, 
Eoger  Bacon,  the  doctor  mirabilis,  deserves  special  mention,2 
as  he  is  in  many  respects  the  equal  of  the  great  Dominican, 
Albertus  Magnus.  His  chief  services  to  science  are  in  the 
domain  of  physics,  chemistry  and  medicine,  rather  than  in 
that  of  the  descriptive  natural  sciences.  Considering  the  age 
in  which  he  lived,  he  had  wonderfully  advanced  opinions 
regarding  physiology.  Much  attention  has  been  paid  to  Bacon 
by  Emile  Charles,3  who  declares  that  the  results  stated  in  his 

1  In   the  printed   text   of   the   lecture   there  is  a  query  after  the  word 
rat,  but  having  had  some  correspondence  with  Stadler,  I  infer  from  a  letter 
dated  December  4,  1905,  that  the  query  ought  to  be  omitted,  as  Albert  the 
Great  was  really  the  first  to  describe  the  rat. 

2  See  Dr.   H.    Felder,   0.    Cap.    Geschichte  der  wissenschaftlichen  Studien 
im  Franziskanerorden  bis  um  die  Mitte  des  13  Jahrhunderts,  Freiburg  i.   B., 
1904,  pp.  379-402. 

3  Roger  Bacon,  sa  vie,  ses  outrages,  ses  doctrines  d'apres  des  textes  inedites, 
Paris,  1861. 


work  '  De  vegetabilibus '  surpass  those  of  Albert  the  Great.  We 
receive  an  impression  of  something  quite  modern,  in  fact 
almost  anti-vitalistic,  when  the  mediaeval  Franciscan  speaks 
thus  of  the  relation  in  which  chemistry  (which  he  calls  alchimia 
speculative!,)  stands  to  the  other  natural  sciences  : l 

Because  students  are  not  acquainted  with  this  science,  they 
also  know  nothing  of  its  bearing  upon  natural  history,  for  instance, 
the  origin  of  living  creatures,  plants,  animals  and  men.  .  .  .  For 
the  constitution  of  the  bodies  of  men,  animals  and  plants  depends 
upon  an  intermingling  of  elements  and  fluids,  and  proceeds  in 
accordance  with  laws  similar  to  those  governing  inanimate  bodies. 
Consequently  whoever  is  ignorant  of  chemistry,  cannot  possibly 
understand  the  other  natural  sciences,  nor  theoretical  and  practical 
medicine.  . 


As  soon  as  the  age  of  discoveries  began  in  modern  times, 
much  more  interest  was  taken  in  the  study  of  nature,  and  the 
tree  of  biological  knowledge  put  forth  one  branch  after  another, 
all  of  which  were  full  of  vigorous  life.  In  our  historical  sketch 
we  must  follow  this  process  of  division,  and  we  will  begin  by 
considering  the  growth  of  systematic  classification,  leaving 
for  the  present  the  development  of  some  other  branches.2 

It  was  natural  that  external  differences  in  form  should  be 
the  first  things  to  attract  the  attention  of  a  student,  in  the 
case  both  of  plants  and  of  animals  ;  later  on  he  tried  to  learn 
something  about  the  mysteries  of  their  constituents,  of  their 
configuration,  and  of  the  vital  phenomena  of  living  organisms. 
It  was  natural,  therefore,  for  systematic  zoology  and  that 
scientia  amabilis,  systematic  botany,  to  develop  earlier  than 
the  other  branches  of  biology.  We  cannot  do  more  than 
mention  the  chief  pioneers  in  systematics.  Edward  Wotton, 
an  Englishman,  wrote  in  1552  a  book  called  '  De  differentiis 

1  Opus  tertium,  c.    12,  ed.  Brewer,  39  :  Et  quia  haec  scientia  ignoratur 
a  vulgo  studentium,  necesse  est  ut  ignorent  omnia  quae  sequuntur  de  rebus 
naturalibus  ;  scilicet  de  gencratione  animatorura,  et  vegetabilium  et  animalium 
et  hominum  :  quia  ignoratis  prioribus  necesse  est  ignorari  quae  posteriora  sunt. 
Generatio  enim  hominum  et  brutorum  et  vegetabilium  est  ex  elementis  et 
humoribus    et   communicat   cum   generatione   rerum   inanimatarum.     Unde 
propter  ignorantiam  istius  scientiae  non  potest  sciri  naturalis  philosophia 
vulgata  nee  speculativa  medicina  nee  per  consequens  practica.  .  .  . 

2  Cf.  R.  Burckhardt,  '  Zur  Geschichte  der  biologischen  Systematik,'  Bale, 
1903  (Verhandlungen  der  Naturf.  Gesellschaft  in  Basel,  XVI). 


animalium,'  in  which  he  returned  to  Aristotle's  system,  which 
he  developed  by  adding  to  it  the  group  of  zoophytes.  Another 
Englishman,  John  Kay  (1628-1705), l  denned  the  Aristotelian 
idea  of  species  more  clearly.  His  works, '  Methodus  plantarum 
nova'  (1682)  and  'Historia  plantarum'  (1686-1704),  are  very 
important  in  systematic  botany,  whilst  his  synopses  of  various 
classes  of  animals,  especially  of  quadrupeds  and  snakes  (1693), 
mark  an  epoch  in  systematic  zoology.  In  this  way  Kay,  the 
son  of  an  English  blacksmith,  facilitated  the  work  done  by  the 
great  Swedish  knight  Karl  v.  Linne  (Linna3us),  who  was  born 
in  1707,  being  the  son  of  a  Protestant  pastor  in  Rdshult.  A 
year  after  the  birth  of  Linna3us  died  his  chief  forerunner  in 
botanical  research,  the  eminent  Frenchman,  Joseph  Pitton  de 
Tournefort  (1656-1708),  who  in  his  '  Elements  de  botanique 
ou  methode  pour  connaitre  les  plantes  '  laid  the  foundation  of 
our  present  classification  of  plants. 

The  work  of  Linnaeus  (1707-78)  marks  a  fresh  stage  in 
the  growth  of  the  tree  of  biological  knowledge,  and  caused  it 
to  become  a  vigorous  trunk  with  many  branches.  Under  his 
influence  it  grew  strong  enough  to  support  the  wealth  of 
offshoots  which  were  destined  to  spring  from  it  during  the 
nineteenth  century.  He  made  many  journeys  to  Central 
Europe  in  order  to  study  the  chief  collections  of  his  day,  and 
with  unflagging  industry  he  acquired  the  material  for  his 
great  work,  the  '  Systema  naturae,'  which  stands  alone  of  its 
kind  and  is  of  the  utmost  importance  in  the  history  of  biology. 
The  first  edition  appeared  in  1735,  the  fifteenth  (which  was 
the  last  revised  by  Linnaeus  himself)  in  1766-8.  The  most 
complete  and  best  known  is  the  seventeenth  edition  of  the 
1  Animal  Kingdom  '  brought  out  by  Gmelin,  1788-92. 

The  chief  value  of  the '  Systema  naturae '  lies  not  so  much  in 
the  fact  that  Linnaeus  has  in  it  formed  systematic  groups  of 
all  previously  described  varieties  of  animals  and  plants,  adding 
many  fresh  ones  to  those  already  known,  but  rather  in  his 
having  introduced  in  his  binary  nomenclature  a  fixed  scientific 
terminology,  so  that  exact  statements  of  laconic  brevity 
thenceforth  took  the  place  of  long-winded  descriptions.  This 
work  of  Linnaeus  had  as  important  a  bearing  upon  the  develop- 
ment of  descriptive  natural  science,  as  the  introduction  of  a 

1  Ray  died  on  January  17,  1705,  not,  as  is  generally  stated,  in  1704. 


written  language  has  upon  the  development  of  a  nation.  Until 
a  language  possesses  a  grammar  and  a  vocabulary,  it  is  only 
a  scientific  embryo  ;  its  elements  lack  sharpness  and  clearness  ; 
it  has,  so  to  say,  no  framework  to  which  they  can  be  attached 
in  orderly  fashion. 

There  is  no  need  for  a  long  explanation  of  the  binary 
nomenclature.  It  is  enough  to  say  briefly  that  to  every 
species  of  animal  and  plant  a  scientific  double  name  is  assigned, 
consisting  of  a  generic  and  a  specific  name,  both  latinised 
in  form,  and  as  these  names  are  constant,  universally  current 
and  unchanging,  they  are  free  from  arbitrary  fluctuations  in 
use,  such  as  are  of  common  occurrence  in  the  case  of  popular 
names.  To  the  generic  name,  which  is  a  noun,  the  differentia 
specified  is  added  by  connecting  with  it  the  specific  name,  which 
is  an  adjective.  Canis  familiaris,  Carabus  auratus,  and  Carabus 
nitens  may  be  taken  as  typical  examples.  Whoever  gives  a 
name  of  this  kind  adds  a  concise  description  of  the  animal  to 
serve  as  a  means  of  identifying  its  species,  and  a  writer  using 
the  name  appends  to  it  in  abbreviated  form  that  of  the  author 
who  first  gave  it  and  described  the  animal  in  question, 
so  that,  when  in  future  any  one  reads  Carabus  auratus,  L. 
(Linnaeus),  he  knows  exactly  once  for  all  what  form  it  is 
intended  to  designate.  In  this  way  a  name  such  as  Carabus 
auratus,  L.,  becomes  a  generally  recognised  scientific  appellation, 
leaving  nothing  to  be  desired  in  the  way  of  clearness  and 
simplicity.  Through  the  use  of  the  binary  nomenclature, 
the  whole  zoological  and  botanical  system  has  been  reduced 
to  a  classified  catalogue,  well  arranged  and  visible  at  a  glance, 
and  in  devising  it  Linnaeus  conferred  an  inestimable  boon 
upon  biology.  The  inspiration  thus  in  so  simple  a  manner 
to  arrange  logically  the  vast  multiplicity  of  forms  in  the  animal 
and  vegetable  kingdoms  is  like  Columbus'  egg — before  Linnaeus 
appeared,  no  one  knew  how  it  could  be  made  to  stand  at  all, 
but  after  Linnaeus  had  once  for  all  set  it  upright,  no  one  had 
anything  to  do  but  to  follow  his  example. 

On  account  of  his  '  Systema  naturae '  Linnaeus  is  to 
be  reckoned  as  the  founder  of  modern  systematic  science. 
His  system  of  nomenclature  is  still  the  standard  one,  and  will 
probably  continue  to  be  so.  The  laws  of  zoological  nomencla- 
ture, as  elaborated  at  the  close  of  the  nineteenth  century  by  a 



committee,  specially  appointed  for  the  purpose  at  recent 
zoological  congresses,1  and  universally  adopted  in  scientific 
circles,  are  only  a  logical  carrying  out  and  detailed  specialisa- 
tion of  the  principles  laid  down  by  Linnaeus.  At  the  annual 
meeting  of  the  German  Zoological  Society  in  1891,  it  was 
decided  to  appoint  a  committee  to  lay  down  rules  securing 
uniformity  in  zoological  nomenclature.3  In  order  to  have  a 
firm  basis  on  which  to  decide  disputed  points  of  priority,  the 
German  Zoological  Society  caused  a  reprint  of  the  tenth 
edition  of  Linnaeus' '  Systema  naturae '  to  be  issued,  thus  marking 
the  year  1758,  in  which  the  tenth  edition  first  appeared,  as  the 
date  when  systematic  zoology  originated,  and  fixing  as  the 
standard  generic  names  those  used  at  that  time  by  Linnaeus. 

The  International  Botanical  Association  is  now  dealing 
with  the  question  of  botanical  nomenclature  at  the  Inter- 
national Botanical  Congresses,  of  which  the  first  was  held  in 
Paris  in  1900,  and  the  second  at  Vienna  in  1905. 

Linnaeus'  '  Systema  naturae '  is  a  monumental  work,  such  as 
could  be  accomplished  only  at  one  period,  at  least  by  a  single 
individual.  By  means  of  the  further  development  of  systematic 
zoology  and  botany,  effected  by  a  closer  study  of  European 
fauna  and  flora,  as  well  as  by  the  exploration  of  foreign  coun- 
tries, which  has  supplied  a  boundless  and  ever-increasing 
wealth  of  material,  systematic  science  has  now  attained 
such  gigantic  proportions,  that  no  single  human  intellect,  not 
even  the  genius  of  an  Aristotle,  would  be  capable  of  grasping 
and  assimilating  it  in  all  its  details.  In  the  year  1901  the 
total  number  of  species  of  animals  known  to  science  amounted 
to  at  least  500,000,  of  which  more  than  half  are  insects.  In 
giving  the  number  of  species  of  beetle  at  100,000  we  are  probably 
rather  understating  it.  In  the  vegetable  kingdom  it  is 
estimated  that  there  are  about  200,000  species  scientifically 
described,  divided  into  11,000  genera — there  are  50,000 
species  of  cryptogams  alone. 

1  Regies  de  la  Nomenclature  des  ttres  organises,  adoptees  par  les  Congres 
Internationaux    de    Zoologie,    Paris,    1889  et   Moskou,    1892    (Paris,    1895); 
Report  on  rules   of  Zoological  Nomenclature,   to  be   submitted   to   the   fourth 
International    Congress   at   Cambridge   by   the   International    Commission  for 
Zoological  Nomenclature  (Leipzig,  1898) ;  Regies  de  la  Nomenclature  Zoologique 
adoptees  par  le  cinquieme  Congres  International  de  Zoologie  (Berlin,  1901). 

2  Verhandlungen  der  Deutschen  Zoolog.   Oesellschaft,   1891,   p.   47  ;     1892, 
p.  13  ;    1893,  p.  89,  &e. 


In  order  to  collect  the  enormous  mass  of  information  on 
systematic  zoology  which  is  now  scattered  in  numberless 
articles  in  numberless  scientific  periodicals  and  books, 
the  German  Zoological  Society  determined,  at  their  first 
general  assembly  in  1891,  to  issue  a  great  systematic  work 
entitled  'Species  animalium  recentium'  or  'Das  Tierreich' 
('  The  Animal  Kingdom  '),  which  should  contain  systematically 
arranged  descriptions  of  all  the  existent  kinds  of  animals  as  far  as 
they  are  at  present  known.  This  great  plan,  which  in  Linnaeus' 
time  was  not  beyond  the  power  of  one  man,  can  now  only  be 
carried  out  by  a  scientific  society  having  at  its  disposal  many 
workers  and  abundant  means  ;  and  even  so  it  is  doubtful 
whether  the  new  '  Animal  Kingdom  '  will  be  completed  by  the 
year  2000.  I  have  made  a  careful  calculation  with  regard 
to  entomological  literature,  the  results  of  which  will  perhaps 
be  of  interest  here.1 

Every  number  of  the  work  is  to  be  arranged  according  to 
the  same  detailed  plan,  therefore,  from  the  nineteen  numbers 
that  had  appeared  in  1894,  we  can  form  some  idea  of  the 
probable  extent  of  the  whole.3  Assuming  that  the  same 
method  is  followed  in  subsequent  numbers  as  in  those  that 
have  already  appeared,  for  the  Order  of  Coleoptera  alone, 
according  to  a  moderate  estimate,  111  volumes  of  500  pages 
each  will  be  required,  for  the  whole  class  of  insects  at  least  300 
volumes  of  500  pages,  and  for  the  whole  animal  kingdom  at 
least  500  volumes  of  500  pages.  These  500  volumes  would 
contain  approximately  15,625  signatures,  so  that  if  the  work 
is  to  be  completed  in  100  years,  156  must  be  issued  yearly. 
But,  as  a  matter  of  fact,  since  1897  on  an  average  less  than 
fifty  signatures  have  appeared  each  year. 

It  is  not  my  wish  to  take  a  pessimistic  view  of  the  matter, 
but  to  give  the  reader  some  idea  of  the  advance  made  in 
biological  knowledge.  Let  us  hope,  therefore,  that  the  whole 
enormous  task  will  be  completed  within  a  reasonable  period, 
before  the  '  Twilight  of  the  Gods  '  foretold  by  Wala  sets  in,  for 

1  Cf .  my  discussion  of  the  first  numbers  of  the  '  Tierreich '  in  Natur  und 
Offenbarung,  XLIII  (1897),  508  ;   XLIV  (1898),  635. 

2  Cf.  the  annual  reports  submitted  to  the  meetings  of  the  German  Zoological 
Society  by  Professor  F.  E.  Schulze,  the  general  editor.     The  publication  of  the 
work  has  now  been  undertaken  by  the  Berlin  Academy   of  Science.     By 
the  summer  of  1905  twenty- three  numbers  had  appeared. 


this  would  probably  be  a  twilight  of  zoologists  also  ;  let  us 
hope  that  the  zoology  of  the  future  will  derive  much  pleasure 
and  satisfaction  from  this  creation  of  the  German  Zoological 
Society  ;  in  any  case,  the  calculation  I  have  made  will  serve 
to  give  my  readers  some  approximate  conception  of  the  enor- 
mous strides  made  by  systematic  zoology  in  the  course  of  the 
nineteenth  century. 

Modern  botanists,  too,  have  undertaken  the  publication 
of  vast  systematic  works,  continuing  the  enormous  task  of 
systematisation  on  Linna3us'  principles.  One  of  these  works 
is  '  Die  natiirlichen  Pflanzenf amilien  nebst  ihren  Gattungen  und 
wichtigeren  Arten,'  von  A.  Engler  und  K.  Prantl  ('  The  natural 
families  of  plants  together  with  their  genera  and  more  im- 
portant species,'  by  A.  Engler  and  K.  Prantl).  The  Phanero- 
gams were  completed  before  the  end  of  the  nineteenth  century, 
in  a  space  of  about  twenty  years,  and  are  contained  in  eleven 
stately  volumes,  but  the  Cryptogams  are  not  finished  yet. 

Another  huge  work  on  botany,  the  counterpart  of  the 
*  Species  animalium  recentium,'  is  being  brought  out  by  A.  Engler 
for  the  Eoyal  Academy  of  Science  in  Berlin,  under  the  title 
'  Eegni  vegetabilis  conspectus.'  It  has  been 'appearing  at 
intervals  since  1900,  and  numerous  collaborators  in  all  parts  of 
the  world  are  engaged  on  it.  We  may  trust  that  there  are  fewer 
hindrances  in  the  way  of  its  completion  than  in  that  of  the 
'  Tierreich,'  in  the  case  of  which  the  enormous  class  of  insects 
presents  great  difficulties,  though  it  is  to  be  hoped  that  these 
will  eventually  be  overcome. 

There  is  one  respect,  however,  in  which  the  systematic 
advance  of  modern  zoology  and  botany  is  not  on  the  lines 
of  Linna3us'  '  Systema  naturae.'  Linnaeus  was  unable  to 
avoid  using  external  differences  as  the  distinctive  marks  of  his 
systematic  groups,  and  in  this  way  he  was  led  to  unite  in  an 
artificial  system  forms  that  bore  no  natural  relationship  to 
one  another.  In  describing  and  classifying  plants  and  animals 
modern  systematic  science  can  avail  itself  of  the  assistance 
of  other  biological  sciences,  especially  of  anatomy  and  of 
morphogeny,  or  the  history  of  individual  development,  and 
thus  it  attains  to  a  more  or  less  successful  natural  classifica- 
tion of  organic  forms.  In  spite  of  this  difference,  however, 
it  is  true  that  modern  systematic  science  is  based  upon 


Linnaeus  and  his  '  Systema  naturae,'  for  without  this  achieve- 
ment of  his  powerful  intellect  we  should  at  the  present  time 
have  had  no  natural  systems  of  plants  and  animals. 

The  fact  that  the  German  Zoological  Society  regarded  it  as 
necessary  to  issue  a  fresh  edition  of  Linnaeus' '  Systema  natura'e,' 
and  to  undertake  the  publication  of  a  great  work  on  systematic 
zoology  on  the  same  lines,  is  testimony  enough  to  the  import- 
ance of  systematics  or  the  science  of  classification  in  the  develop- 
ment of  biological  knowledge.  It  shows  at  the  same  time  how 
deeply  indebted  the  representatives  of  modern  science  are  to 
Linnaeus,  and  it  is  to  be  regretted  that  in  some  of  the  more 
recent  books  on  zoology  Linnaeus  is  mentioned  as  the  founder 
of  the  '  unintelligent  zoology  of  species,'  and  this  in  more  or 
less  plain  language.1 

To  a  certain  class  of  Haeckelists,  systematic  science  seems 
like  an  inconvenient  old  man,  who  threatens  to  check  them 
in  their  bold  intellectual  tricks  and  fantastic  speculations, 
precisely  because  the  actual  multitude  of  forms  in  the  animal 
world  does  not  coincide  with  their  ideas,  and  because  they  are 
too  impatient  to  be  willing  to  master  the  subject-matter  of 

1  R.  Hertwig  is  however  justified  in  stating  in  his  Lehrbuch  der  Zoologie, 
7th  edit.,  1905,  p.  9,  that  post-Linnsean  zoologists,  and  especially  entomologists, 
have  made  it  their  sole  aim  to  describe  the  greatest  possible  number  of  new 
species,  making  quantity  rather  than  quality  the  measure  of  their  achievements. 
Unfortunately,  even  at  the  present  day  this  class  of  pseudo-systematic 
biologists  is  not  quite  extinct,  and  there  are  still  some  who  flood  the  scientific 
periodicals  with  superficial  or  even  '  provisional '  descriptions,  and  thereby 
put  obstacles  in  the  way  of  studying  some  groups  of  animals,  for  other,  more 
thorough  workers,  who  can  make  nothing  of  these  superficial  descriptions, 
are  hindered  by  being  obliged  by  the  law  of  priority  to  take  them 
all  into  account.  An  almost  incredible  story  is  told  of  a  l  scientific 
worker '  who  was  employed  about  fifty  years  ago  at  a  great  museum,  and  was 
paid  £1  for  each  new  genus  and  Is.  for  each  new  species  that  he  established. 
In  order  to  work  more  quickly,  he  had  two  bags  beside  him,  one  filled  with 
Greek  and  the  other  with  Latin  names.  If  he  wanted  a  name  for  a 
new  genus,  he  put  his  hand  into  the  Greek  bag  and  pulled  out  a  name  hap- 
hazard, and  bestowed  it  upon  his  genus.  If,  on  the  other  hand,  he  wanted 
a  name  for  a  new  species,  he  had  recourse  to  the  Latin  bag,  and  labelled  it 
with  the  first  adjective  that  he  caught  up.  It  can  easily  be  imagined  how 
applicable  the  new  names  thus  assigned  were  to  the  genera  and  species,  and 
the  descriptions  which  he  appended  as  '  original '  to  these  names  were  equally 
suitable.  Such  work  as  this  was  really  '  unintelligent  zoology  of  species,' 
but  it  would  be  unfair  to  regard  zoology  of  species  as  responsible  for  such  lack 
of  intelligence.  There  are  excrescences  in  every  branch  of  knowledge,  and 
they  do  not  occur  more  frequently  in  the  systematic  zoology  of  the  Linnaean 
school  than  in  the  modern  doctrine  of  evolution.  Ernst  Haeckel's  famous 
book,  The  Riddle  of  the  Universe,  affords  a  striking  instance  of  unintelligent 
blunders  on  the  part  of  the  Darwinian  supporters  of  this  doctrine.  See  my 
criticism  of  the  same  in  Stimmen  aus  Maria- Loach,  LX,  1901,  p.  428,  &o. 


systematics  before  beginning  their  speculations.  They  com- 
pletely forget  that  but  for  this  stern  old  father  they  would 
have  no  existence  at  all. 

Mere  systematics  is  certainly  by  no  means  the  ideal  of  bio- 
logical knowledge  ;  it  is  not  an  end  in  itself,  but  is  only  an 
indispensable  aid  to  biological  research.  It  bears  the  same 
relation  to  the  other  biological  sciences  as  the  dry  heart-wood 
of  a  tree  bears  to  its  tissues  permeated  by  life-giving  sap  ;  it 
forms  the  skeleton  or  scaffolding  for  other  sciences.  But  just 
as  in  the  human  body  the  eye  has  no  right  to  reproach  the 
bones  of  the  foot  for  not  responding  to  the  vibrations  of  ether, 
so  modern  morphology  and  morphogeny  ought  not  to  look 
down  upon  systematics  for  not  perceiving  many  things  that 
these  branches  of  science  can  discover.  In  science,  as  in  the 
living  organism,  the  principle  of  the  subdivision  of  labour 
holds  good,  and  the  greater  the  perfection  attained  by  any 
science,  and  the  more  numerous  its  departments,  the  more 
indispensable  is  it  to  distinguish  clearly  the  subject-matter 
with  which  each  single  subdivision  deals,  if  any  solid  progress 
is  to  be  made. 

Let  us  apply  this  consideration,  the  truth  of  which  no 
modern  scientific  man  will  question,  to  Linnaeus'  position 
with  regard  to  biology.  Scientific-  classification  or  systematics 
was  his  speciality,  and  it  was  a  boon  to  science  that  Linnaeus 
with  his  vast  intellect  devoted  himself  to  it  rather  than  to 
anatomy  and  physiology,  for  the  formation  of  a  strong 
systematic  science  was  the  first  and  most  necessary  starting 
point  for  all  the  other  branches  of  biological  science,  if  they 
were  to  thrive  at  all.  Without  it  zoology  and  botany  would 
have  remained  a  hopeless  chaos  of  forms,  through  which  no  one 
could  have  found  his  way. 

In  order  to  produce  a  g*eat  systematic  work  like  Linnaeus' 
'  Systema  naturae,'  even  at  that  time  a  man  was  required  who 
should  devote  his  whole  ability  to  this  end,  for  otherwise  it 
would  have  been  unattainable.  When  his  pygmy  successors, 
who  have  inherited  the  achievements  of  his  genius,  reproach  the 
great  Linnaeus  with  being  merely  a  one-sided  systematist,  they 
show  themselves  to  be  both  short-sighted  and  ungrateful. 




Malpighi  and  Swammerdam's  anatomy  of  insects  (p.  26).  Bichat's 
Comparative  Anatomy  (p.  26).  G.  Cuvier's  services  to  the  various 
branches  of  zoology  (p.  27). 


The  invention  of  the  microscope  (p.  29).  The  discovery  of  the  cell  and 
nucleus  (p.  30).  Schwann  and  Schleiden's  theory  of  cells  and  its 
subsequent  development  (p.  32).  The  meaning  of  protoplasm  (p.  33). 


General  and  particular  methods  for  definite  microscopical  purposes  (p.  34). 


OF  A  DIMINUTIVE  FLY  (Termitoxenia)  (p.  37); 

and  in  investigating  genuine  inquiline  relationship  in  the  case  of  guests 
among  ants  and  termites  (p.  44). 


Cytologists  of  various  nationalities  (/>.  45). 


WE  have  already  shown  how  Aristotle  may  justly  be  regarded 
as  the  founder  of  modern  systematics,1  and  he  may  with  equal 
right  be  called  the  first  morphologist  in  the  modern  sense, 
because  he  carried  on  a  comparative  study  of  the  varieties  of 
form  among  animals.  Aristotle  laid  the  foundation  of  the 
science  of  morphology  in  his  work  'De  partibus  animaliurn,'  and 
Galen  (131-201  A.D.)  continued  what  Aristotle  had  begun,  for 
his  famous  work  on  human  anatomy  is  based  chiefly  upon  post- 
mortem investigations  on  the  higher  animals,  and  so  should  be 
called  animal  rather  than  human  anatomy.  The  real  originator 
of  human  anatomy  was  Vesalius  (1514-64),  who  dissected 
human  bodies,  and  thus  was  able  to  correct  many  errors  arising 
out  of  Galen's  studies  of  animals. 

1  Cf.  also  on  this  subject  Professor  R.  Burckhardt,  'Zur  Geschichte  der 
biologischen  Systematik '  (reprinted  from  the  Verhandlungen  der  Naturf.  Gesell- 
schaft  in  Basel,  XVI,  1903,  pp.  388-440). 



Marco  Aurelio  Severino  (1580-1656),  a  Calabrian,  was  the 
author  of  the  first  book  on  general  anatomy.  It  was  published 
in  Nuremberg  in  1645,  and  bears  the  title  :  '  Zootomia  Demo- 
critaea,  id  est  anatome  generalis  totius  animalium  opificii  libris 
quinque  distincta.'  Severino  treats  the  '  lower  animals  '  in  a 
very  curt  fashion  ;  they  fare  better  at  the  hands  of  writers 
towards  the  close  of  the  seventeenth  century.  Marcello 
Malpighi,  a  Bolognese  physician  (1628-94),  wrote  a  '  Dis- 
sertatio  epistolica  de  bombyce '  (1669)  on  the  anatomy  of  the 
silkworm,  and  this  work  opened  the  way  to  the  anatomical 
study  of  insects,  for  the  discovery  of  the  Malpighian  tubes, 
of  the  heart,  nervous  system,  tracheae,  &c.,  for  the  first  time 
revealed  insects  as  organic  masterpieces,  whose  wonderful 
construction  is  scarcely  inferior  in  perfection  to  that  of  the 
higher  animals,  and  is  more  worthy  of  admiration,  because 
of  its  diminutive  size. 

Johann  Swammerdam  (1637-85),  who  lived  at  Amsterdam, 
in  his  '  Bijbel  der  natuure'  (Biblia  naturae),  published  1737-8, 
describes  with  astonishing  accuracy  the  internal  structure  of 
bees,  ephemera,  snails,  &c. ;  and  whoever  is  acquainted  with 
the  excellent  anatomical  discussion  of  the  larva  of  the  goat- 
moth,  published  in  1760  by  Pieter  Lyonet  of  Maastricht, 
cannot  fail  to  recognise  its  merits  even  at  the  present  time, 
when  we  can  avail  ourselves  of  greatly  improved  instruments 
and  technical  methods  in  dealing  with  the  same  subject. 

The  great  scientists  mentioned '  above  inaugurated  a  new 
era  in  anatomical  knowledge,  yet  morphology  was  still  not  a 
systematically  organised  science,  but  only  a  collection  of 
interesting  monographs.  It  was  raised  to  the  rank  of  a  special 
science  at  the  beginning  of  the  nineteenth  century,  by  Bichat, 
a  Frenchman,  who  introduced  the  idea  of  systems  of  organs 
and  systems  of  tissues.  Bichat's '  Traite  des  membres  en  general ' 
(1800)  and  his  '  Anatomie  generale'  (1801)  created  comparative 
anatomy,  for  he  divided  the  constituent  parts  of  the  bodies 
of  animals  into  organs  and  tissues,  and  into  systems  of  organs 
and  tissues,  thus  fixing  a  firm  basis  for  the  comparison  of  the 
constituent  parts  of  various  animals.  It  is  true  that  this  idea 
of  Bichat's  was  not  altogether  new  ;  Aristotle,  Galen,  and 
Albert  the  Great  distinguished  heterogeneous  and  homogeneous 
parts  among  the  constituents  of  the  bodies  of  animals.  The 


heterogeneous  parts  are  the  individual  organs,  the  homo- 
geneous are  the  tissues,  which  may  be  found  in  various  organs, 
and  of  which  the  organs  are  composed. 

A  famous  Italian  anatomist,  Gabriele  Antonio  Fallopius 
(1523-62),  as  early  as  the  sixteenth  century  wrote  '  Tract  at  us 
quinque  de  parti  bus  similibus,'  in  which  he  distinguished  and 
described  a  considerable  number  of  tissues.  In  1767  Bordeu, 
a  Frenchman,  devoted  an  entire  work  to  one  kind  of  tissue, 
viz.  the  mucous  connective  tissue;  his  book  bears  the  title 
'  Eecherches  sur  le  tissu  muqueux  ou  organe  cellulaire.'  Still 
it  was  Bichat  who  first  arranged  the  homogeneous  tissues  as  a 
scientific  whole,  distinguishing  them  from  organs  and  systems 
of  organs.  A  system  of  organs  is  a  complex  of  organs  working 
together  to  discharge  the  same  vital  function  and  so  forming 
one  physiological  whole.  A  system  of  tissues  is  a  complex  of 
tissues  consisting  of  the  same  morphological  elements,  and  so 
forming  one  logical  whole,  from  the  point  of  view  of  compara- 
tive morphology.  Two  examples  will  explain  this  distinction. 
The  digestive  system  in  man  is  a  system  of  organs,  for  it  is 
made  up  of  several  organs  which  unite  to  produce  one  and 
the  same  physiological  result,  though  they  are  formed  of 
various  kinds  of  tissue  ;  for,  in  addition  to  epithelial  tissue, 
both  connective  and  muscular  tissues  enter  into  their  structure.  • 
But  the  glandular  system  in  man  is  a  system  of  tissues,  for  it 
consists  of  essentially  similar  tissues,  viz.  modifications  of  the 
epithelium,  which  serve  very  various  physiological  purposes  ; 
such  are  the  gland  of  the  intestine,  the  renal  gland,  the  salivary 
gland,  the  sweat  gland,  &c.  In  other  cases  the  distinction 
between  a  system  of  organs  and  a  system  of  tissues  is  not  so 
strongly  marked  as  in  those  to  which  I  have  just  referred. 
For  instance,  when  we  speak  of  the  nervous  system  of  man, 
we  are  alluding  to  both  a  system  of  organs  and  a  system  of 
tissues.  Nevertheless,  in  theory  the  two  systems  are  totally 
distinct  even  here.1 

A  far  greater  man,  and  one  who  had  much  more  influence 
on  the  development  of  comparative  morphology,  was  Georges 
Cuvier  (1769-1832).  He  was  born  at  Mompelgard  and  educated 

1  Textbooks  on  zoology  treat  chiefly  of  systems  of  organs,  and  those  on 
histology  chiefly  of  systems  of  tissues,  therefore  a  writer  on  zoology  is  apt 
to  ignore  the  histological  point  of  view,  and  vice  versa,  which  is  disastrous 
to  perspicuity. 


at  the  Karlsakademie  in  Stuttgart.  Whilst  he  was  professor 
of  comparative  anatomy  at  the  Jardin  des  Plantes  in  Paris, 
he  published  numerous  important  works.  In  1812  he  estab- 
lished a  new  classification  of  the  animal  kingdom,  which  is 
known  as  Cuvier's  Theory  of  Types,  and  is  based  upon  the 
anatomical  comparison  of  the  various  groups  of  animals. 
According  to  it  animals  are  divided  with  reference  to  their 
structure  into  four  main  classes,  which  Cuvier  called  em- 
branchements,  but  Blainville  subsequently  substituted  the 
name  types.  These  are  vertebrata,  mollusca,  articulata,  and 
radiata.  Cuvier's  Theory  of  Types  was  expanded  and  elaborated 
by  Karl  Ernst  von  Baer  (1792-1876),  an  Esthonian,  the  founder 
of  comparative  embryology,  whose  theory  of  germinal  layers 
reduced  the  embryology  of  animals  to  a  scientific  system. 

Cuvier's  Theory  of  Types  was  not  by  any  means  his  sole 
contribution  towards  the  development  of  modern  zoology. 
His  comprehensive  work  'Le  regne  animal'  (1816), l  in  the 
compilation  of  which  he  was  assisted  by  many  collaborators, 
is  the  most  important  achievement  in  the  domain  of  systematics 
since  the  time  of  Linna3us.  His  '  Histoire  des  sciences  naturelles,' 
published  after  his  death  in  Paris  (1841-5),  as  E.  Burckhardt 
aptly  remarks,2  presents  the  history  of  zoology  and  the  natural 
sciences  in  one  vast  frame,  and  is  a  monumental  work  of  wide 
scope.  Cuvier  devoted  much  attention  also  to  fossil  animals, 
and  between  1795  and  1812  he  brought  out  several  works  on 
the  subject,  laying  down  definite  morphological  principles  to 
be  followed  in  comparing  fossils  with  still  existing  animals  of 
the  zoological  system,  and  he  thus  became  one  of  the  chief 
founders  of  modern  palaeontology.  His  chief  service  to  com- 
parative biology  was  that  he  established  the  law  of  correlation, 
i.e.  he  was  the  first  to  formulate  the  regular  connexion  of  the 
organs  of  any  animal  with  one  another,  and  with  its  habits 
and  environment.  Although  Cuvier  did  not  regard  as  essential 
the  variations  of  form  within  his  four  great  types,  he  was  an 
adherent  of  the  theory  of  permanence,  and  in  1798  for  the 
first  time  he  gave  a  clear  concise  statement  of  the  meaning  of 
the  *  systematic  species,'  a  definition  that  still  holds  good. 
His  views  on  the  permanence  of  species  brought  him  into 

1  The  fourth  edition  in  eleven  volumes  appeared  1836-49. 

2  '  Zur  Geschichte  der  biologischen  Systematik,'  390. 


conflict  with  his  contemporaries,  Jean  Lamarck  and  Etienne 
and  Isidore  Geoffroy  St.  Hilaire,  who  upheld  the  transmutation 
theory.  The  scientific  struggle  carried  on  by  the  members 
of  the  French  Academy  ended  for  a  time  in  the  victory  "of 
Cuvier's  opinion,  but  we  shall  have  to  recur  in  the  ninth 
chapter  to  the  further  history  of  the  theory  of  evolution. 


Hitherto,  in  speaking  of  the  development  of  anatomy,  we 
have  referred  chiefly  to  macroscopic  anatomy,  which  is  not 
dependent  upon  the  microscope  ;  it  is,  however,  to  this  instru- 
ment that  most  of  the  progress  made  by  modern  morphology 
is  due.1 

It  was  invented  some  hundreds  of  years  ago,  but  not  until 
the  nineteenth  century  did  the  real  age  of  microscopical 
research  begin.  As  early  as  the  year  1100  the  Arab,  Alhazen 
ben  Alhazen,  described  the  magnifying  power  of  a  convex 
lens.  The  English  Franciscan,  Koger  Bacon,  who  lived  1214- 
1294,  and  whom  we  have  already  mentioned  (p.  16),  seems 
to  have  constructed  complicated  optical  instruments.  He  is 
said  to  have  ground  a  piece  of  glass  so  that  people  saw  wonder- 
ful things  in  it,  and  ascribed  its  action  to  the  power  of  the 
devil.  If  this  glass  deserves  to  be  called  a  microscope,  the 
honour  of  inventing  this  instrument  would  have  to  be  ascribed 
to  Roger  Bacon,  but  various  nations  claim  to  have  given  birth 
to  the  inventor  of  it.  The  Italians  say  that  either  Galileo  or 
Malpighi  invented  it,  but  most  people  consider  two  Dutchmen, 
Hans  and  Zacharias  Janssen  (1590),  to  be  more  justly  entitled 
to  the  credit  of  the  invention.  The  name  '  microscope  '  was 
first  applied  to  the  new  instrument  by  Giovanni  Faber  in  Rome 
in  1625,  and  many  improvements  in  it  were  made  about  1646 
by  the  astronomer  Francesco  Fontana  in  Naples.  Malpighi 
and  Swammerdam  certainly  used  the  microscope  in  their 
scientific  work,  and  the  Dutchman  Anton  Leeuwenhoek  of 
Delft  (1632-1723),  the  '  Father  of  the  Microscope  '  as  Schlater 
calls  him,  used  it  in  examining  the  ova  and  stings  of  bees,  and 
many  other  things  connected  with  the  anatomy  of  insects. 

1  Cf.  Dr.  J.  Peiser,  '  Die  Mikroskopie  einst  und  jetzt,'  in  Natur  und  Schule, 
IV,  1905,  parts  10,  11. 


By  its  aid  he  discovered  infusoria,  and  drew  the  attention  of 
scientific  men  to  a  new  world  of  diminutive  creatures,  our 
knowledge  of  which  was  greatly  increased  by  Christian  Gott- 
fried Ehrenberg  in  the  middle  of  the  nineteenth  century.  By 
means  of  the  microscope  Leeuwenhoek  was  enabled  to  discover 
the  red-blood  corpuscles  and  the  transverse  striation  of  the 
muscular  apparatus,  and  Hamm  to  perceive  spermatozoa, 
the  key  to  those  mysterious  problems  of  heredity  which 
the  greatest  biologists  of  the  present  day  are  so  eager  to 

Thus  we  see  that  microscopical  anatomy  made  steady 
progress,  and  advanced  towards  the  marvellous  triumphs 
of  modern  histology  and  cytology.  It  was,  however,  a  long 
time  before  scientific  men  generally  made  use  of  the  microscope  ; 
it  is  a  surprising  fact  that  even  in  1800  it  was  altogether 
neglected  by  Bichat,  to  wiiom  we  have  already  referred  as  the 
founder  of  comparative  anatomy.  Consequently  he  could  give 
no  account  of  cells,  the  smallest  constituents  of  animal  tissues, 
although  they  had  long  before  been  recognised  by  other  scien- 
tific men  who  used  the  microscope. 

Who  discovered  cells  and  the  structure  of  organic  tissues 
out  of  cells  ?  In  plants  it  is  much  easier  to  find  the  cells, 
as  they  possess,  as  a  rule,  a  more  independent  existence  in 
plants  than  in  animals.  It  is  therefore  only  natural  that  cells 
were  discovered  first  in  botany.  An  Englishman,  Robert 
Hooke,  gave  cells  their  name  because  of  their  resemblance 
to  the  cells  of  the  honeycomb.  In  his  '  Micrographia,'  which 
appeared  in  1667,  he  gave  the  first  illustration  of  a  plant  cell, 
or  rather  cell-wall.  The  figure  represents  a  bit  of  cork,  along 
which  lengthwise  run  rows  of  black  specks  or  cells.  Hooke's 
purpose  in  speaking  of  cells  was  not  so  much  to  add  to  the 
scientific  knowledge  of  botany,  as  to  display  the  power  of  his 
microscope,  and  so  it  is  usual  to  ascribe  the  discovery  of  cells 
to  two  other  scholars,  the  Italian  Malpighi  (1674),  whom  we 
have  already  mentioned,  and  the  Englishman  Nehemiah  Grew 
(1682).  Their  works  on  this  subject  appeared  at  almost  the 
same  time,  a  few  years  after  Hooke's  '  Micrographia.'  Ninety 
years  elapsed  before  another  great  scientist  continued  their 
work.  In  1759  Kaspar  Friedrich  Wolff  published  his  remark- 
able book  '  Theoria  generations,'  in  which  he  propounded  new 


ideas  on  morphogeny,  and  threw  much  light  on  the  morphology 
of  organisms.  His  descriptions  and  illustrations  show  plainly 
that  he  had  studied  the  cells  in  both  animal  and  vegetable 
tissues  ;  he  calls  those  in  the  former  '  globules  '  or  '  spheres  '  and 
those  in  the  latter  '  utriculi '  or  '  cells.'  With  regard  to  botany, 
clear  evidence  that  the  vascular  system  of  plants  consists  of 
cells  was  adduced  by  Treviranus  in  his  work  '  Vom  inwendigen 
Bau  der  Gewachse '  ('  The  internal  structure  of  vegetables'), 
1808.  The  honour  of  having  been  the  first  to  discover  and 
mention  the  nucleus  of  the  living  cell  is  generally  ascribed  to 
an  Italian-Tyrolese,  Abbe  Felice  Fontana,  1781.  However, 
H.  Bolsius,  S.  J.,1  has  recently  proved  that  the  discovery  was 
made  by  Leeuwenhoek,  the  Dutch  scientist  already  mentioned, 
in  1686,  about  a  century  earlier. 

The  English  botanist,  Robert  Brown,  was  the  first  to 
discover  (1833)  the  regular  significance  of  the  nucleus  in  its 
relation  to  the  cell,  and  for  this  reason  many  people  regard 
him  as  the  real  discoverer  of  the  nucleus.2 

It  was  not  until  Joseph  von  Fraunhofer  in  1807  constructed 
the  first  achromatic  lenses,  and  thus  greatly  increased  the 
capabilities  of  the  microscope,  that  modern  cytology  was 
able  to  develop.  It  is  a  remarkable  fact  that  just  at  this  time 
(1809)  Mirbel,  a  Frenchman,  began  again  to  apply  the  name 
'  cell '  to  the  smallest  elements  in  living  organisms  ;  Malpighi's 
word  utriculus  had  long  taken  its  place,  but  now,  at  the  dawn 
of  modern  cytology,  the  old  name  was  revived,  which  Hooke 
had  given  to  these  organic  elements  150  years  before.  The 
word  '  cell '  is  still  in  use,  in  spite  of  various  attempts  to 
substitute  some  more  modern  name,  such  as  protoblast  (Kolliker) 
and  plastid  (Haeckel).  The  study  of  the  organic  tissues 
composed  of  cells  was  first  designated  Histology  by  Karl 
Mayer  in  Bonn  in  1819.  Germany  is  therefore  the  real  home  of 
both  histology  and  cytology,  and,  as  even  the  French  scientists 
acknowledge,  both  have  grown  and  developed  chiefly  in 

1  Antoni  von  Leeuwenhoek  et  Felix  Fontana,  '  Essai  historique  sur  le  revela- 
teur  du  noyau  collulaire,'  Rome,  1903  (Memorie  delta  Pontificia  Accademia 
Romano,  de.i  Nuovi  Lincei,  XXI). 

'2  Cf.  0.  Herfcwig's  Allgemeine  Biologie  (1906),  pp.  5  and  27.  Hertwig's 
account  of  the  history  of  the  cell  theory  is  very  valuable,  pp.  4,  &c. 

3  Cf.  M.  Duval,  Precis  d1  Histologie,  Paris,  1900,  p.  12. 


Everyone  who  has  ever  opened  a  modern  book  on 
zoology  or  botany  must  know  the  names  of  Schleiden  and 

Matthias  Jakob  Schleiden,  bom  1804  in  Hamburg,  became 
the  founder  of  modern  botanical  cytology  when,  in  1838,  he 
published  his  '  Beitrage  zur  Phytogenesis  '  in  Miiller's  '  Archiv.' l 
The  zoologist,  Theodor  Schwann,  born  1810  in  Neuss,  applied 
the  same  principles  to  animal  tissues  in  1839,  when  he  pub- 
lished his  '  Mikroskopische  Untersuchungen  iiber  die  Uberein- 
stimmung  in  der  Struktur  und  dem  Wachstum  der  Tiere  und 
Pflanzen,'  2  and  he  added  so  much  to  Schleiden's  work  that  we 
generally  speak  of  Schwann-Schleiden's  theory  of  cells,  or 

In  the  case  of  every  object  of  sense  perception,  human 
knowledge  invariably  proceeds  from  the  exterior  to  the  interior, 
from  the  shell  to  the  kernel,  and  this  is  true  of  our  knowledge  of 
cells.  The  dry  walls  of  dead  plant  cells  were  what  Hooke 
called  cells  250  years  ago.  Malpighi  also  studied  particularly 
the  plant-cell,  which  is,  as  a  rule,  much  larger  and  has  thicker 
and  more  conspicuous  walls  than  the  animal  cell,  and  hence 
it  became  the  custom  to  regard  the  cellular  membrane  as  the 
essential  part  of  the  cell.  Malpighi  and  Wolff  represented  the 
cell  as  being  practically  an  empty  tube  or  bag — and  this  was 
equivalent  to  mistaking  a  snail  shell  for  a  snail.  Schleiden 
and  Schwann  had  a  deeper  insight  into  the  truth,  for  they  had 
better  aids  to  research  at  their  disposal ;  they  discovered 
that  each  tube  or  bag  is  filled  with  a  fluid,  and  they  noticed 
the  nucleus,  though  this  had  been  discovered  long  before. 
Their  opinion  was  that  the  cell  is  a  little  vessel  filled  with 
fluid  in  which  a  nucleus  is  suspended.  Subsequent  examina- 
tion of  young  cells  has  shown  that  they  have  no  real  walls,  and 
the  membrane  appears  to  be  an  accidental  part  of  the  cell, 
and  thus  the  scientific  idea  of  the  cell  advanced  to  the  third 
stage,  at  which  it  still  practically  remains.  Franz  Ley  dig  in 

1  Cf.    Jos.    Rompel,    S.J.,    '  Der   Botaniker   Matthias   Jakob    Schleiden  ' 
(1804-81),  in  Natur  und  Offenbarung,  I  (1904),  parts  4-7;  see  especially  pp. 

2  '  Microscopical  researches  into  the  accordance  in  the  structure  and  growth 
of  animals  and  plants. ' 

3  The  botanists  Treviranus  and  Meyen  ought  to  be  mentioned  as  having 
prepared  the  way  for  Schleiden.    Their  works  were  published  in  1808  and  1830 


1857  l  and  Max  Schultze  in  1861  3  denned  a  cell  as  a  mass  of 
living  protoplasm  containing  one  or  more  nuclei. 

The  fluid  contents  of  the  cell  were  called  protoplasm  by 
Hugo  von  Mohl  in  1846,  and  the  name  has  been  universally 
adopted,  for  it  conveys  an  idea  fundamental  in  biological 
research.3  Dujardin  in  1835  had  named  the  same  substance 
sarkode,  but  no  one  now  uses  this  word. 

Von  Mohl  drew  the  attention  of  scientists  to  the  movements 
of  protoplasm  within  the  cells  of  plants,  but  they  had  been 
noticed  long  before  by  Bonaventura  Corti  (1774)  and  C.  L. 
Treviranus  (1807),  and  described  as  '  rotatory  movements  of 
the  cellular  fluid.' 

At  this  point  the  question  naturally  arises  :  What  are  the 
chemical  constituents  of  protoplasm  ?  In  the  first  part  of  his 
*  Studien  iiber  das  Protoplasma '  (1881),  J.  Keinke  describes  it 
as  *  a  mixture  of  numerous  organic  compounds.'  Von  Hanstein, 
however,  in  1879  defined  protoplasm  as  an  albuminous  com- 
pound or  a  mixture  of  albuminous  compounds,  and  he  proposed 
to  call  it  protoplastin.  In  his  '  Lehrbuch  der  Zoologie,'  R. 
Hertwig  says  in  a  resigned  way  that  we  must  acknowledge  our 
inability  to  determine  the  chemical  characteristics  of  proto- 
plasm. '  It  is  not  known  whether  protoplasm  is  a  definite 
chemical  body,  which  from  its  constitution  is  capable  of  infinite 
variation,  or  whether  it  is  a  varying  mixture  of  different 
chemical  substances.  So,  also,  we  are  by  no  means  certain 
whether  or  not  these  substances  (as  one  is  inclined  to  believe) 
belong  to  those  other  enigmatical  substances,  the  proteids.  We 
can  only  say  that  the  constitution  of  protoplasm  must,  with 

1  The  year  1859  or  1861  is  generally  given  as  the  date  when  cytology  entered 
upon  its  third  stage,  therefore  I  will  quote  here  a  passage  from  Leydig's  Lehrbuch 
der  Histologie  des  Menschen  und  der  Tiere,  published  at  Frankfurt  a.  M.  in 
1857.     He  writes  as  follows  (p.  9)  :    'To  the  morphological  conception  of  a 
cell  belongs  a  more  or  less  soft  substance,  originally  almost  globular  in  form, 
containing  a  central  body  called  the  nucleus.'     This,  therefore,  according  to 
Leydig's  opinion  in  1857  was  the  essence  of  the  cell — he  had  already  discarded 
the  membrane  as  non-essential — for  he  continues  :    '  The  substance  of  the 
cell  frequently  hardens  so  as  to  form  a  more  or  less  independent  outer  layer 
or  membrane,  and  when  this  takes  place  the  cell  is  technically  said  to  consist 
of  membrane,  substance,  and  nucleus.' 

2  *  Uber  Muskelkorperchen  und  das,  was  man  eine  Zelle  zu  nennen  habe ' 
(Archiv  fur  Anatomie  und  Physiologic,  1861). 

3  Cf.  0.  Hertwig,  Allgemeine  Biologie,  p.  7,  &c.,  for  the  history  of  the 
protoplasm  theory ;    p.    12,   &c.,  for  investigations  regarding  the  meaning 
and  nature  of  protoplasm. 



a  certain  degree  of  homogeneity,  have  a  very  extraordinary 
diversity.' l 

We  may  be  satisfied  to  endorse  J.  Keinke's  2  remark  that 
our  conception  of  protoplasm  has  always  been  morphological, 
i.e.  all  we  know  about  it  is  that  it  forms  the  primary  substance 
common  to  every  living  cell.  A  detailed  account  of  all  the 
information  hitherto  acquired  on  the  subject  of  the  chemical 
composition  of  protoplasm,  as  well  as  on  that  of  the  organisa- 
tion of  the  cell  and  nucleus,  and  their  reciprocal  chemical 
relations,  will  be  found  in  E.  B.  Wilson's  '  The  Cell  in  Develop- 
ment and  Inheritance,'  New  York,  1902,  chapter  vii ;  also  in  0. 
Hertwig's  '  Allgemeine  Biologie,'  Jena,  1906,  chapter  ii,  pp.  12, 
&c.  On  pp.  18  et  seq.  Hertwig  has  shown  very  clearly  that 
the  discovery  of  the  substance  and  process  of  life  is  a  vital 
problem,  and  not  merely  an  affair  of  chemistry  and  physics. 
This  subject  will  be  discussed  more  fully  in  Chapters  VII  and 

Our  knowledge  of  tissues  and  cells  has  been  vastly  increased 
by  means  of  microscopical  research  since  the  middle  of  the 
nineteenth  century.  The  names  of  the  scientific  men  distin- 
guished in  this  branch  of  research  would  make  a  long  list ;  we 
can  mention  only  the  most  eminent — Henle,  Gerlach,  Keichert, 
Eemak,  Leydig  and  Kolliker — some  of  the  more  recent 
zoologists  will  be  noticed  later  on.  Botanists  have  been  no 
less  zealous  than  zoologists  in  studying  cells  under  the  micro- 
scope. We  may  refer  to  W.  Hofmeister,  A.  Zimmermann,  de 
Bary  and  Sachs,  as  well  as  to  the  more  recent  students — 
Pfeffer,  Wiesner,  and  Strasburger. 


Microscopical  research  has  been  greatly  facilitated  by  the 
discovery  of  the  modern  methods  of  chemical  colouring. 

As  soon  as  definite  colouring  matters  were  applied  to  animal 
and  vegetable  tissues,  their  structure  became  more  plainly 
visible,  and  the  structure  of  the  cell  itself  was  revealed,  for 
the  nucleus  was  found  to  absorb  readily  certain  colouring 

1  English  translation,  1903,  p.  61. 

-'  Einleitung  in  die  theoretische  Biologie,  Berlin,  1901,  p.  221. 


matters  which  do  not  affect  the  protoplasm  of  the  cell.  The 
nucleus  was  then  seen  to  contain  some  darker  coloured  granules 
or  filaments  or  nucleoli,  which  suggested  the  idea  that  the 
nucleus  was  not  a  simple  but  a  composite  body.  In  the  same 
way  there  appeared  in  the  protoplasm  darker  coloured  granules 
or  a  network  of  filaments  against  a  lighter  background,  and  the 
observation  of  these  led  to  the  discovery  of  the  cell  framework. 
When  the  colouring  process  was  applied  to  cells  and  nuclei 
in  course  of  division,  pictures  of  wonderful  beauty  were  revealed, 
from  which  the  laws  of  the  division  of  the  nucleus  and  of 
fertilisation  were  learnt. 

Gerlach  in  1858  first  used  carmine  as  a  stain  for  microscopical 
purposes,  and  since  his  time  the  number  and  variety  of  colouring 
methods  have  increased  almost  indefinitely.  Gerlach  used 
carminate  of  ammonia,  others  have  employed  alum-carmine, 
borax-carmine  or  carmalum,  picro-carmine,  &c. 

The  carmine  stains  were,  however,  discarded  in  favour  of 
haematoxylin,  an  excellent  stain  prepared  from  logwood 
(Haematoxylon  campechianum),  which  is  applied  in  various 
solutions  and  combinations,  and  is  still  much  used  in  micro- 
scopical work.  The  double  stains  obtained  by  using  haema- 
toxylin in  conjunction  with  eosin  or  Congo  red  or  saffranin 
have  lasted  admirably,  and  have  produced  beautiful  and 
instructive  plates,  so  that  haematoxylin  has  not  yet  been 
displaced  by  its  numerous  rivals  prepared  from  coal-tar,  and 
known  as  aniline  dyes.  The  colouring  methods  just  mentioned, 
and  especially  the  use  of  haematoxylin  and  its  combinations, 
are  of  universal  application,  and  can  be  employed  for  almost 
all  histological  purposes,  but  there  are  also  certain  special 
methods  of  staining  particular  tissues,  especially  those  of  the 
nerves.  Golgi,  Kamon  y  Cajal,  and  Eanvier  used  solutions  of 
nitrate  of  silver,  chromate  of  silver,  and  formic  acid  with 
chloride  of  gold,  in  their  attempts  to  overthrow  the  long- 
established  theory  of  a  central  nervous  system,  and  thus 
extended  our  knowledge  of  ganglion  cells  and  their  processes. 

When  Waldeyer  formulated  his  theory  of  neurones  in  1891, 
and  when  soon  after  the  theory  of  fibrils  was  put  forward  in 
opposition  to  it,1  the  chief  arguments  adduced  in  this  scientific 

1  At  the  seventy- second  meeting  of  German  naturalists  and  physicians  at 
Aix-la-Chapelle  in  1900,  a  lively  discussion  of  the  two  theories  took  place. 

D  2 


contest  were  supplied  by  observations  on  the  nervous  system, 
rendered  possible  by  the  use  of  stains, — methods  which  Apathy, 
Bethe,  Nissl,  Held,  Bielschowsky  and  others  have  carried  to 
the  utmost  perfection.  The  anatomical  and  physiological 
study  of  nerves  owes  much  to  Ehrlich,  Eetzius  and  others, 
who  have  succeeded  in  staining  the  nervous  system  of  a  living 
animal  with  methyl  blue,  so  that  it  has  become  possible  to  trace 
the  action  of  the  finest  fibres  and  terminations  of  the  nerves. 

Quite  recently  Carnoy  and  other  cytologists  at  Louvain 
have  used  methyl  green,  and  have  shown  it  to  be  of  great 
service  in  the  development  of  biology,  for  it  gives  a  vivid 
colour  to  the  nucleus  of  a  cell  still  living,  thus  rendering  visible 
the  most  minute  details  of  its  structure. 

As  special  stains,  used  in  studying  the  stages  of  division 
of  the  nucleus  in  the  process  of  mitosis,  we  may  mention  parti- 
cularly Heidenhain's  use  of  iron  alum  with  haematoxylin  and 
Plattner's  metallic  nuclear  black. 

All  these  colouring  methods  would  avail  but  little,  however, 
if  scientists  had  not  at  their  disposal  a  means  of  cutting  organic 
tissues,  as  well  as  entire  animals  and  plants,  after  artificially 
hardening  them,  into  layers  so  thin  that  light  can  penetrate 
them  and  make  their  wonderful  construction  visible  under 
the  microscope.  The  art  of  cutting  sections  is  as  indispensable 
as  the  art  of  staining,  and  it  is  by  means  of  both  in  conjunction 
that  microscopic  anatomy  has  been  enabled  to  make  its 
extraordinary  progress  in  recent  times.  It  owes  the  one  to 
chemistry,  and  the  other  to  modern  mechanics,  which  created 
the  microtome  and  placed  it  at  the  service  of  biology. 

The  microtome  is  a  mechanical  apparatus  which  passes  an 
extremely  sharp  knife  in  a  definite  direction  over  an  object 
embedded  in  paraffin  or  celloidin  or  some  similar  embedding 
substance,  and  at  the  same  time  a  movable  plate  provided  with 
a  scale  automatically  regulates  the  thickness  of  each  section. 

As  at  each  turn  of  the  plate,  about  a  given  angle,  the  knife 
is  lowered,  for  instance,  y^mm.,  or  (in  other  microtomes)  the 
object  is  raised  Yoo"mm'>  a  skilful  worker  is  able  to  obtain  an 

M.  Verworn  supported  the  theory  of  neurones  in  his  lectures,  '  Das  Neuron  in 
Anatomie  und  Physiologie'  (reprinted  at  Leipzig,  1901).  See  also  Fr.  Nissl, 
Die  Neuronentheorie  und  ihre  Anh'dnqer,  Jena,  1903  ;  M.  Wolff,  '  Neue  Beitrage 
zur  Kenntnis  des  Neurons '  (Biolog.  Zentralblatt,  1905,  Nos.  20-22) ;  Wasmann- 
Gemelli,  La  Biologia  Moderna,  Florence,  1906,  p.  44  note. 


unbroken  series  of  sections,  each  y^mm.  in  thickness.     In  the 
same  way  he  can  obtain  sections  of  -^^mm.,  g^ 

if  he  requires  them.  The  microtomes  most  generally  used  at  the 
present  day  are  those  made  by  E.  Jung  in  Heidelberg.  Micro- 
tomes on  another  system  were  devised  by  Professor  Hatschek 
and  made  by  Jensen  in  Prague  ;  in  these  the  knife  does  not 
move  up  and  down  along  an  inclined  surface,  as  it  does  in 
Jung's  apparatus,  but  it  moves  backwards  and  forwards  over 
a  horizontal  surface.  With  the  latter  I  have  succeeded  better 
than  with  the  former,  and  have  even  prepared  very  thin  and 
regular  sections  cut  through  the  hard  chitin  integument  of 
beetles  and  other  insects.  There  are  also  lever  microtomes, 
English  microtomes  with  a  pointed  spindle,  and  Minot's  new 
American  microtomes  intended  to  cut  sections  of  larger 
objects.  The  construction  of  these  ingenious  instruments  has 
in  the  last  few  years  become  a  special  branch  of  mechanics, 
and  interesting  accounts  of  their  great  perfection  may  be  found 
in  the  illustrated  price-lists  issued  by  E.  Jung  and  Walb  in 
Heidelberg,  Eeichert  in  Vienna,  and  others. 


(Termitoxenia.)     (PLATE  V) 

I  should  like  to  illustrate  the  great  advance  made  in  bio- 
logical research  through  the  adoption  of  modern  methods  of 
staining  and  cutting  sections,  and  my  illustration,  derived 
from  my  own  work,  will  take  my  readers  out  of  the  gloom  of 
theories  into  the  cheerful  atmosphere  of  practical  results. 

I  am  at  this  moment  studying  some  extremely  small  insects 
only  1-2  mm.  in  length,  belonging  to  the  order  of  Diptera. 
They  have  a  relatively  enormous  white  abdomen,  and  in  the 
course  of  the  last  few  years  have  been  found  in  the  nests  of 
termites  in  South  Africa,  the  Soudan  and  India,  by  G.  D. 
Haviland,  Dr.  Hans  Brauns,  J.  B.  Heim,  J.  Assmuth,  S.J., 
and  Y.  Tragardh.i 

1  In  subsequent  chapters  I  shall  have  occasion  to  refer  repeatedly  to  this 
remarkable  fly.  belonging  to  the  family  of  Termitoxeniidae.  An  account  of  it 
is  given  in  Chapter  X,  '  Theory  of  Permanence  or  Theory  of  Descent,'  and 
illustrations  will  be  found  on  Plate  V. 


Diptera  of  the  normal  type  have  two  wings,  but  in  their 
stead  this  little  creature  (which  I  have  described  under  the 
generic  name  Termitoxenia) l  has  peculiar  appendages  to  the 
thorax  (Plate  V,  figs.  1,  2,  4,  5)  which  are  morphologically 
homologous  with  wings,  but  have  actually  so  developed  as  to 
serve  quite  other  purposes  than  that  of  flight,  for  which  their 
narrow,  club-shaped  or  hooked  form  and  their  horny  structure 
render  them  altogether  unsuitable.  They  are,  however,  well 
adapted  to  perform  a  number  of  new  functions,  closely  connected 
with  the  insect's  habit  of  living  among  the  termites.  The 
appendages  to  the  thorax  of  the  Termitoxenia  serve  as  organs  of 
transport,  by  which  these  little  inquilines  are  picked  up  and 
carried  about  by  their  hosts  ;  they  serve  to  maintain  the 
fly's  equilibrium  and  enable  it  to  balance  itself  when  it  walks, 
as  otherwise  the  enormous  size  of  its  body  would  render  walk- 
ing very  difficult ;  they  are  sense  organs,  supplying  the  creature 
with  a  great  many  percepts  by  way  of  touch  ;  they  are  organs 
of  exudation,  through  which  it  emits  a  volatile  element  in 
its  blood  as  a  pleasing  stimulant  to  the  greed  of  its 
hosts ;  finally  they  resemble  supplementary  spiracles,  that  to 
some  extent  are  like  the  tracheal  gills  of  the  insect's  earliest 
aquatic  ancestors. 

These  little  termitophile  Diptera  are  indeed  a  store-house 
of  anomalies,  whether  we  consider  them  from  the  point  of  view 
of  morphologists,  anatomists,  evolutionists,  or  biologists. 
They  are  exceptions  to  the  laws  of  entomology.  They  are 
not  merely  Diptera  without  wings,  but  they  are  flies  without 
the  larval  and  pupal  stages,  and  are  actually  insects  having 
neither  male  nor  female  ! 

In  order  to  shorten  the  lengthy  and  complete  process  of 
metamorphosis  undergone  by  other  Diptera,  the  Termitoxenia 
lays  comparatively  enormous  eggs,  from  which  is  hatched  not 
a  larva,  as  is  the  case  with  other  flies,  but  a  perfect  insect, 

1  *  Termitoxenia,  em  neues  fliigelloses,  physogastres  Dipterengenus  ain 
Termitennestern,'  Part  I  (Zeitschrift  fur  wissenschaftliche  Zoologie,  LXVII, 
1900,  pp.  599-618  with  plate  XXXIII) ;  Part  II  (ibid.  LXXX,  1901,  pp. 
289-98) ;  '  Zur  naheren  Kenntnis  der  termitophilen  Dipterengattung 
Termitoxenia  '  (Verhandl.  des  V.  internationalen  Zoologenkongresses  zu  Berlin, 
August  1901,  pp.  852-72  with  one  plate) ;  '  Die  Thorakalanhange  der  Ter- 
mitoxeniidae,  ihr  Bau,  ihre  imaginale  Entwicklung  und  phylogenetische 
Bedeutung'  (Verhandl.  der  deutschen  Zoolog.  Gesellschaft,  1903,  pp.  113-120, 
with  plates  II  and  III). 


the  imago  form,  still  in  a  stenogastric  or  thin-bodied  con- 
dition. To  compensate  for  the  absence  of  metamorphosis, 
the  Termitoxenia,  as  imago,  undergoes  a  postembryonic  de- 
velopment, for  its  organs  of  generation,  especially  the  single- 
tubed  ovaries,  its  fat-body,  consisting  of  large  cells  joined 
together  end  to  end,  its  abdominal  muscular  system,  and  even 
the  outer  skin  of  the  abdomen,  receive  their  final  form  only  in 
the  course  of  a  long  process  of  growth.  Each  of  these  insects 
is  moreover  a  complete  hermaphrodite,  there  are  no  distinct 
males  and  females  at  all.  The  youngest  imagines  have  some 
quite  undeveloped  ovaries,  such  as  occur  in  the  larvae  of  other 
Diptera,  but  even  in  the  youngest  specimens  the  male  generative 
glands  and  the  bundles  of  spermatozoa  connected  with  them 
are  well  marked,  although  they  subsequently  become  atrophied, 
when  the  spermatozoa  have  ripened,  whilst  the  ovaries  develop. 
We  have,  therefore,  here  an  instance  of  what  is  called  prot- 
andric  hermaphroditism,  which  regularly  allows  first  the 
male  and  then  the  female  generative  glands  to  develop  in  the 
same  individual,  so  that  the  Termitoxenia  is  something  quite 
unique  in  insect  biology. 

It  is  most  interesting  to  trace  the  development  of  the 
ovaries.  (See  Plate  V,  fig.  6.)  Each  one  consists  of  a  single 
egg-tube — a  phenomenon  long  sought  in  vain  among  insects 
by  the  upholders  of  the  theory  of  evolution,  until  Grassi 
discovered  it  occurring  in  the  very  rudimentary  ground-flea 
(podura),  belonging  to  the  genus  Campodea. 

This  single  egg-tube  on  each  side  of  the  Termitoxenia' $ 
body  is,  in  the  case  of  the  youngest  specimens,  merely  one 
single  long  terminal  chamber,  filled  with  apparently  un- 
differentiated  little  nuclei.1 

In  course  of  time  the  egg-tube  contracts  in  between  the 
eggs,  and  forms  a  long  series  of  ovarian  chambers,  those  at  the 
lower  end  of  the  ovary  being  the  largest.  In  each  of  these 
chambers  the  elements  of  the  ovary  differentiate  themselves 
into  nutritive  cells  and  true  egg-cells,  so  that  each  chamber 
eventually  contains  several  large  cells,  one  of  which  develops 

1  I  use  the  word  *  apparently  '  advisedly,  for  in  one  of  his  recent  works 
('Untersuchungen  iiber  die  Histologie  des  Insektenovariums,'  in  the  Zoologische 
Jahrb'ucher,  Section  for  Anatomy,  1903,  part  1),  Gross  has  proved  that  the 
epithelial  cells  and  those  that  eventually  become  germ-cells  differ  from  one 
another  even  in  the  terminal  chamber. 


more  rapidly  than  the  rest  and  becomes  the  egg.  The  other 
cells  in  the  same  chamber  serve  as  its  food,  or,  in  scientific 
language,  a  fusion  takes  place  of  the  egg-cell  with  the  nutri- 
tive cells,  the  substance  of  the  latter  being  gradually  absorbed 
into  that  of  the  former,  and  transformed  into  tiny  yolk- 
capsules  collected  round  the  germinal  vesicle  of  the  young  egg. 
Thus  the  egg  is  nourished  and  it  continues  to  grow  until  it 
occupies  about  a  quarter  of  the  entire  abdomen  of  the  full- 
grown  insect.  (Plate  V,  fig.  6  ov.)  By  this  time  it  has  taken 
up  enough  yolk-material  to  serve  for  the  whole  embryonic 
development  until  it  reaches  the  stage  of  imago,  when  it  must 
make  its  own  way  in  the  world.  It  is  fertilised,  and,  passing 
along  the  ovarian  duct,  it  is  laid  among  the  eggs  of  the 

The  history  of  the  development  of  a  fly  belonging  to  the 
sub-genus  Termitomyia  is  somewhat  different,  but  still  more 
extraordinary.  In  this  case  the  egg,  whilst  still  within  the 
parent's  body,  becomes  an  embryo,  wilich  develops  until 
it  reaches  the  form  of  a  stenogastric  imago.  Therefore  this 
sub-genus  lays  no  eggs  at  all,  but  brings  forth  its  young 
alive.  These  viviparous  insects  are  a  worthy  contrast  to  the 
oviparous  mammals,  such  as  the  ornithorhynchus  and  the 
Australian  ant-eating  Echidna. 

There  is  a  regular  correlation  between  all  the  points  on  which 
the  remarkable  anatomy  and  development  of  the  Termitoxenia 
differ  from  those  usual  among  insects.  The  fact  that  each 
ovary  has  only  one  egg-tube  facilitates  the  formation  of  eggs 
few  in  number,  but  large  and  rich  in  yolk.  The  large  size  and 
richness  in  yolk  of  the  eggs  render  the  omission  of  the  larval 
and  pupal  stages  possible,  and  so  the  whole  process  of  develop- 
ment is  conveniently  shortened  and  simplified,  and  the  imago 
is  produced  out  of  the  egg  or  rather  out  of  the  embryo. 

Moreover,  in  the  case  of  the  Termitoxenia,  the  complicated 
process  of  assigning  sex  to  the  individual  is  simplified  in  a 
form  that  is  perfectly  ideal  for  insects,  as  each  individual 
fulfils  both  functions.  And  all  these  wonderful  peculiarities 
in  the  morphology,  development,  and  biology  of  the  Termito- 
xenia, its  physogastria  and  its  ametabolia,  its  growth  as  an 
imago  and  its  hermaphroditism,  the  shape  of  its  appendages 
to  the  thorax  and  the  formation  of  the  parts  of  its  mouth — 


for  it  has  a  long  proboscis  for  sucking  the  tender,  juicy  young 
of  the  termites — all  these  are  closely  connected  with  and 
dependent  upon  the  affection  of  these  Diptera  for  the  termites  ! 

And  how,  it  may  be  asked,  do  we  know  all  this  ?  Have 
observations  been  made  in  India  and  Africa  regarding  the 
habits  of  these  diminutive  creatures,  and  has  their  development 
been  studied  for  years  in  artificial  nests  of  termites  ?  By  no 
means.  The  discoverers  of  the  six  known  varieties  of  Termito- 
xenia  merely  established  the  fact  that  they  always  are  found 
in  the  nests  of  certain  kinds  of  termites  and  among  their  eggs 
and  larvae.  The  inquilines  and  their  hosts  were  sent  to  me 
in  alcohol  or  formol.  But  the  further  question  arises,  how 
can  it  be  possible,  in  that  case,  to  make  such  definite  and 
apparently  rash  statements  as  to  the  habits  of  these  creatures  ? 
They  are  so  small,  that  even  a  powerful  magnifying  glass 
scarcely  enables  us  to  distinguish  the  details  of  their  exterior 
configuration  ;  even  under  the  microscope  it  is  difficult  to 
make  out  the  halteres  or  balancers,  which  are  placed  behind 
the  thoracic  appendages,  and  prove  that  the  latter  morpho- 
logically correspond  to  the  wings  of  Diptera  and  do  not  point 
to  a  coalescence  of  wings  and  halteres. 

What  scientific  evidence  is  there,  then,  in  support  of  the 
account  just  given  of  the  anatomy,  development,  and  biology 
of  Termitoxenia  ? 

The  account  is  based-  on  the  results  obtained  by  modern 
methods  of  using  stains  and  cutting  sections.  The  series  of 
sections  of  Termitoxenia  supply  us  with  material  for  studying 
its  anatomy,  development,  and  biology. 

So  far  I  have  obtained  by  means  of  the  microtome  complete 
series  of  sections  of  sixty  specimens  of  five  species  of  Termito- 
xeniidae  of  various  ages,  and  I  have  also  cut  sections  of  a 
number  of  eggs  of  various  species  ;  as  a  stain  I  have  generally 
used  a  double  preparation  of  haematoxylin  (Delafield's  method) 
and  eosin.1 

The  total  number  of  sections  thus  prepared  amounts  to 
10,000.  Each  specimen  submitted  to  microscopical  examina- 
tion furnishes  a  series  of  from  80  to  200  sections  of  y^  mm.  in 
thickness  ;  the  number  varies  according  as  the  sections  are 

1  Or  a  double  stain  obtained  by  using  haemalum  (Meyer's  method)  and 
orange  eosin,  &c. 



longitudinal  or  transverse.  Each  series  of  sections  therefore 
forms  a  book  of  from  80  to  200  pages,  on  which  are  recorded 
in  unbroken  sequence  the  whole  exterior  and  interior  morpho- 
logy of  the  specimen,  and  this  record  is  legible  under  the 
microscope.  If  the  sections  of  various  kinds  of  Termitoxenia  at 
different  ages,  and  also  of  their  respective  eggs,  are  compared 
with  one  another,  the  morphological  volumes  come  to  form 
a  library  containing  an  account  of  the  Termitoxenia' $  develop- 
ment. As,  however,  almost  every  point  in  the  anatomy 

i.     JT.    sr  JF.    v.    H   m    m. 



















































IT    m    or   v. 


FIG.  1. — Scheme  of  a  series  of  sections  of  Termitoxenia  Heimi  Wasm. 

and  development  of  these  tiny  creatures  is  of  significance  in 
their  habits,  this  library  supplies  also  trustworthy  information 
for  their  whole  biology. 

The  accompanying  illustration  (fig.  1)  represents  a  series 
of  sagittal  sections  of  Termitoxenia  Assmuihi.  It  consists  of 
the  longitudinal  sections  of  specimen  No.  13  of  this  variety, 
arranged  upon  two  slides  (i  and  ii).  The  Koman  numerals  on 
each  slide  refer  to  the  sequence  of  the  rows  of  sections,  the 
Arabic  numerals  to  the  sequence  of  the  sections  in  each  row. 
Thus  the  series  begins  with  No.  1  on  the  first  slide  and  ends 
with  No.  96  on  the  second.  No.  49,  the  first  on  the  second 


slide,  is  a  section  cut  from  the  middle  of  the  creature's  body — 
a  photograph  of  it  will  be  found  on  Plate  V,  fig.  6,  at  the  end 
of  the  book. 

I  need  hardly  say  that  a  great  expenditure  of  time  and 
trouble  is  needed,  not  merely  to  make  such  series  of  sections, 
but  far  more  to  study  them  with  success.  The  instances  of 
morphological  and  biological  conformity  to  law,  which  a 
scientist  can  discover,  seem  to  be  written  in  a  mysterious 
cipher,  the  key  to  which  is  found  only  by  careful  study.  No 
one,  therefore,  will  be  astonished  to  hear  that  I  have  spent 
years  on  my  study  of  the  Termitoxenia,  especially  as  I  had 
not  only  to  describe  my  microscopical  results  in  words,  but 
to  reproduce  them  by  means  of  drawings  or  photographs 
upon  a  series  of  carefully  executed  plates.1 

The  marvellous  beauty  of  the  various  sections  is  no  less 
noticeable  than  their  scientific  value  in  biological  research. 
The  material  for  several  series  of  sections  of  Termitoxenia 
Heimi  and  Assmuthi  was  supplied  me  by  J.  B.  Heim,  S.J., 
Missionary  in  India,  and  J.  Assmuth,  Professor  at  St.  Francis 
Xavier's  College  in  Bombay.  The  creatures  reached  me  in  very 
good  preservation,  having  been  killed  and  hardened  in  a 
mixture  of  alcohol  and  formalin.  The  sections,  stained  with 
haematoxylin  and  eosin,  or  some  similar  double  stain,  are 
so  beautiful  that  they  cannot  fail  to  arouse  admiration  in  any 
one  who  sees  them,  even  in  the  mind  of  one  who  regards 
all  insects  alike  as  '  vermin.'  Eosin  stains  the  protoplasm 
of  the  tissues  various  shades  of  light  red,  whilst  the'  nuclei, 
which  chiefly  serve  to  differentiate  the  various  kinds  of  tissue, 
are  coloured  light  or  dark  blue  by  means  of  haematoxylin  or 
haemalum  ;  the  whole  picture  displays  a  delicacy  of  design 
and  a  beauty  of  colouring  such  as  no  artist's  skill  could  repro- 
duce in  perfection.  The  most  complex  and  most  highly 
coloured  pictures  are  formed  by  sections  showing  the  various 
stages  of  development  in  which  the  mysterious  biological 
processes  of  cell-division,  cell-multiplication,  and  cell-growth — 
those  elementary  functions  of  life — are  most  active. 

Modern  microphotography  will,  perhaps,  succeed  in  fixing 

1  A  fuller  account  of  my  work  will  appear  in  the  Zeitschrijt  fur  wissen- 
schaftliche  Zoologie.  A  resume  of  the  results  obtained  hitherto  was  given  in 
an  address  delivered  at  the  fifth  International  Zoological  Congress  in  Berlin, 
August  1901. 


microscopical  sections  with  all  their  gorgeous  colouring  directly 
upon  photographic  plates.  If  this  is  ever  done,  it  will  be  of 
the  utmost  scientific  importance,  as  the  precise  shades  of 
colour  in  the  nuclei  and  other  parts  of  the  tissues  often  give 
a  trustworthy  clue,  of  great  assistance  in  histological  and 
cytological  research. 

A  learned  professor  of  theology,  on  seeing  some  series  of 
sections  of  the  Termitoxenia,  remarked  very  aptly  that  micro- 
scopical research,  by  means  of  modern  methods  of  staining  and 
cutting  sections,  had  become  a  second  creation,  creatio  secunda, 
revealing  to  us  for  the  first  time  all  the  marvels  which  God 
at  its  first  creation  had  concealed  within  the  body  of  this 
diminutive  fly. 

In  order  to  give  my  readers  a  wider  idea  of  the  application 
of  microscopical  study  to  our  investigations  into  animal 
biology,  the  following  remarks  may  be  added.  Let  us  suppose 
that  some  one  asks :  '  Why  do  ants  and  termites  show  such 
energy  and  pleasure  in  licking  their  "  true  inquilines  "  ?  Upon 
what  does  the  satisfaction  depend  which  they  derive  from  so 
doing  ? ' 

Before  this  question  can  be  answered,  a  reply  must  be  given 
to  another,  viz. :  '  What  tissues  underlie  the  external  exudatory 
organs,  which  lead  to  the  process  of  licking  the  inquilines  ?  ' 
With  a  view  to  answering  this  latter  question  I  have,  in  the 
course  of  the  last  ten  years,  prepared  about  20,000  sections  of 
various  kinds  of  inquilines  among  arits  and  termites  (they  are 
chiefly  beetles),  and  studied  their  tissues  under  the  microscope. 
In  this  way  I  have  arrived  at  the  following  conclusion  : — the 
exudation  of  true  inquilines,  with  which  they  repay  their 
hosts  for  their  hospitality,  is  partly  a  direct  and  partly  an 
indirect  product  of  adipose  tissue  ;  when  it  is  indirect,  it  is 
partly  a  glandular  secretion  and  partly  an  element  in  the  blood 
plasm  of  the  inquiline.1 

We  are  therefore  now  in  a  position  to  divide  the  genuine 
inquilines  among  ants  and  termites  into  various  classes  according 
to  their  exudatory  tissues,  and  thus  have  made  a  perceptible 
step  towards  solving  the  mystery  of  true  guest-relationship. 

1  Articles  on  this  subject  appeared  in  the  Biologisches  Zentralblatt,  1903, 
Nos.  2,  5,  6,  7,  8,  under  the  heading  :  '  Zur  naheren  Kenntnis  des  echten 
Gastverhaltnisses  (Symphilie)  bei  den  Ameisen-  und  Termitengasten.' 



After  this  little  digression  let  us  return  to  the  historical 
development  of  modern  histology  and  cytology. 

Improvements  in  the  microscope  itself,  the  chief  implement 
in  our  research  work,  have  kept  pace  with  the  adoption  of 
better  methods  of  staining  and  cutting  sections. 

As  a  result  of  very  careful  physical  studies,  Abbe  of  Jena 
devised  an  apochromatic  objective,  calculated  exactly  with 
reference  to  its  refractive  and  dispersive  power.  This  was 
worked  out  by  Schott  &  Co.,  in  Jena,  and  then  further  per- 
fected by  Karl  Zeiss,  the  able  optician  in  Jena.  The  apo- 
chromatic objective  has  been  imitated  with  various  degrees 
of  success  by  other  German  and  foreign  firms.  Its  introduc- 
tion, and  that  of  the  corresponding  compensating  ocular  or 
eye-piece,  mark  an  important  stage  in  the  development  of  the 
microscope.  Speaking  from  my  own  personal  experience, 
I  can  safely  assert  that  the  pictures  produced  by  this  system 
of  lenses  are  infinitely  clearer  than  those  produced  by  the 
achromatic  objectives  and  Huygenian  oculars  previously 
in  use.  It  is  now  possible  to  see  every  detail  in  the  structure 
of  tissues  even  when  magnified  1500-2000  times. 

This  advance  in  optical  appliances  has  enabled  modern 
cytologists  to  study  the  most  delicate  construction  of  a  resting 
cell,  as  well  as  the  processes  of  division  and  fertilisation, 
and  to  discover  the  laws  governing  these  most  important 
phenomena  of  life. 

Histology  and  cytology  made  great  progress  during  the 
latter  half  of  the  nineteenth  century  in  other  countries  as  well 
as  in  Germany,  where  they  had  their  birth,  and  where  they 
grew  to  the  rank  of  independent  sciences,  in  consequence  of  the 
research  work  done  by  Schleiden,  Schwann,  Kemak,  Leydig,  . 
and  Max  Schultze. 

I  can  mention  the  names  of  only  a  few  of  the  more  recent 
workers  in  this  department  of  science ;  in  Germany,  besides 
Leydig  and  Max  Schultze,  we  have  Strasburger,  Weismann, 
Memming,  Biitschli,  Henking,  Heidenhain,  Boveri,  A.  Brauer, 
Keinke,  the  two  Hertwigs,  Haecker,  Erlanger,  0.  vom  Eath, 
Schaudinn,  Khumbler,  &c. ;  in  Bohemia,  Eabl ;  in  Hungary, 


Apathy,  who  has  made  nerve-cells  his  special  study ;  in 
Switzerland,  Fol ;  in  France,  Kanvier,  Balbiani,  Giard, 
Maupas,  Kunstler,  Guignard,  Armand  Gautier,  and  Yves 
Delage  ;  in  Belgium,  van  Bambeke,  E.  van  Beneden,  and  the 
great  cytologists  of  the  Catholic  University  of  Louvain,  viz. 
Abbe  Carnoy,  the  author  of  '  Biologie  cellulaire,'  and  his  pupils, 
of  whom  G.  Gilson,  A.  van  Gehuchten,  and  Abbe  Janssens  are 
well  known  through  their  important  publications  ;  in  Spain, 
Bam<5n  y  Cajal ;  in  Italy,  Giardina  ;  in  Great  Britain  and 
Ireland,  A.  Sedgwick,  Moore,  McGregor  and  Dixon  ;  in  Sweden, 
Ketzius  and  Murbeck  ;  in  Bussia,  Kowalevsky,  Tichomirow, 
Nawaschin  and  Sabaschnikoff ;  in  North  America,  Ch.  Sedg- 
wick Minot,  Chittenden,  E.  B.  Wilson,  Th.  H.  Montgomery  and 
Osborn  ;  lastly,  in  Japan,  Chiyomatsu  Ishikawa,  director  of 
the  zoological  institute  of  the  Imperial  University  of  Tokio. 

We  may  therefore  well  say  that  all  civilised  nations  of  the 
present  time  have  contributed  to  the  development  of  modern 
histology  and  cytology.1 

In  order  that  my  readers  may  not  regard  the  Jesuits  as  '  mediaeval 
obscurantists '  trying  to  stem  the  advance  of  science,  I  may  be 
allowed  to  add  that  a  Dutch  Jesuit,  H.  Bolskis,2  has  done  much  to 
increase  our  knowledge  of  the  microscopical  anatomy  of  Hirudines 
or  leaches,  and  has  shown  himself  an  authority  of  the  highest 
rank  on  this  subject.  A  modern  morphological  and  biological 

1  This  is  of  course  true,  not  only  with  regard  to  the  morphology  of  the 
cell,  with  which  we  are  now  chiefly  concerned,  but  also  with  regard  to  its 
vital  phenomena,  especially  the  processes  of  cell  division  and  fertilisation, 
to  which  we  shall  have  to  refer  later.     I  should  like  to  draw  particular  attention 
to  Carney's  Biologie  cellulaire.  1884,  which  unhappily  was  never   completed; 
also  to  Oskar  Hertwig's  Allgemeine  Anatomie  und  Physiologie  der  Zelle,  1893  ; 
and  Max  Verworn's  Allgemeine  Physiologie,  the  third  edition  of  which  appeared 
in  1901,  and  deals  mainly  with  cellular  physiology.     I  regret  that  Verworn's 
work  is  not  altogether  free  from  phrases  suggestive  of  Haeckel's  influence  and 
wanting  in  scientific  dignity.     For  instance,  on  p.  214,  in  speaking  of  par- 
thenogenesis among  the  lower  animals,  he  refers  to  '  the  ancient  legend  of  the 
Immaculate  Conception.'     The  author  seems  to  be  as  far  as  Haeckel  from 
a  comprehension  of  the  dogma  of  the  Immaculate  Conception. 

2  '  Nouvelles   recherches   sur   la   structure   des  organes  segmentaires  des 
Hirudinees,'  1890  ;    '  Les  organes  cilies  des  Hirudinees,'  1891  ;    '  Le  sphincter 
de  la  Nephridie  des  Gnathobdellides,'  1894 ;     *  La  glande  impaire  de  1'Hae- 
mentaria  officinalis,'  1896  ;    '  Recherches  sur  1'organe  cilie  de  1'Haementaria 
officinal!*,'  1900  (this  article  appeared  in  La  Cellule).     I  might  also  mention 
a  number  of  other  articles  which  the  same  author  contributed  to  the  Annales 
de  la  Societe  scientifiqiie  de  Bruxelles,  to  the  Memorie  della  Pontificia  Accademia 
dei  Nuovi  Lincei,  to  the  Zoologischer  Anzeiger  (Leipzig),  and  the  Anatomischer 
Anzeiger  (Jena),  &c. 


work,  universally  regarded  as  a  masterpiece,  has  been  written 
by  J.  Pantel,  a  French  Jesuit,  on  the  larva  of  Thrixion  hali- 
dayanum ;  l  and  no  less  excellent  are  an  anatomical  and  histological 
study  of  the  anal  glands  of  beetles  by  a  Belgian  Jesuit,  Fr.  Dierckx,2 
and  a  biological  and  anatomical  study  of  walking  stick  insects  by  a 
French  Jesuit,  R.  de  Sinety.3 

These  publications,  as  well  as  most  of  the  works  of  Carnoy, 
Gilson,  van  Gehuchten  and  Bolsius,  appeared  in  La  Cellule,  a 
periodical  published  by  the  Cytological  Institute  of  the  Catholic 
University  of  Louvain,  a  society  founded  by  Abbe  Carnoy.  This 
periodical  is  highly  esteemed  by  German  scientists,  and  forms  a 
complete  refutation  of  the  old  fiction  that  Catholics,  and  especially 
those  of  Romance  nations,  must  needs  be  bad  men  of  science.  In 
the  sixth  chapter  I  shall  have  to  refer  to  some  articles  on  the 
chromosomes  in  the  eggs  of  Selachii  and  Teleostei  by  J.  Marechal, 
a  Belgian  Jesuit,  and  among  Italian  scientists,  a  Franciscan, 
Dr.  Fra  Agostino  Gemelli,  has  written  some  excellent  works  on 
anatomy  and  histology  during  the  last  few  years. 

1  '  Le  Thrixion  halidayanum,  Rond. :  Essai  monographique  sur  les  caracteres 
exterieurs,  la  biologie  et  1'anatomie  d'une  larve  parasite  du  groupe  des  Tachi- 
naires,'  1898  (La  Cellule,  XV). 

2 '  fitude  comparee  des  glandes  pygidiennes  chez  les  Carabides  et  les 
Dytiscides,'  1899  (La  Cellule,  XVI)  ;  '  Les  glandes  pygidiennes  des  Coleopteres,' 
2nd  memoire,  1900  (ibid.  XVIII). 

3  Recherches  sur  la  biologie  et  Fanatomie  des  Phasmes,  Lierre,  1901.  This 
work  contains  splendid  illustrations  ;  in  the  eighth  chapter  the  author  dis- 
cusses the  karyokinetic  processes  in  the  spermatogenesis  of  Orthoptera,  a 
subject  of  peculiar  interest  as  throwing  light  on  the  accessory  chromosomes. 




Cells  of  various  shapes  and  dimensions,  giant  and  dwarf  cells  (p.  49). 
Uninuclear  and  multinuclear  cells  (p.  53). 


Hyaloplasm  and  spongioplasm  ;  theories  regarding  the  structure  of  the 
latter;  filar  and  reticular  theory  (p.  56);  alveolar  theory  (p.  57); 
granular  theory  (p,  59).  Reinke  and  Waldeyer's  scheme  for  reconciling 
these  theories  (p.  60). 


Chemical  and  physical  theories  of  colouring  (p.  61).  Fischer's  theory  of  the 
polymorphism  of  protoplasm  (p.  62). 


The  cell  not  a  simple,  but  an  extremely  complex  formation  (p.  65). 


ON  p.  33  we  have  seen  that  Franz  Leydig  in  1857  and  Max 
Schultze  in  1861  denned  the  cell  as  a  small  mass  of  proto- 
plasm containing  one  or  more  nuclei.  This  has  remained  to 
the  present  day  the  fundamental  idea  of  the  cell,  as  we  may  see 
on  referring  to  the  definitions  of  it  given  by  Richard  Hertwig  in 
the  seventh  edition  of  his  '  Lehrbuch  der  Zoologie,' l  Matthias 
Duval  in  the  second  edition  of  his  handbook  of  histology,3  and 
Oskar  Hertwig  in  his  '  Allgemeine  Biologie.'  3  With  regard  to 
this  definition  there  is  almost  unanimous  agreement  on  the  part 
of  the  chief  cytologists  of  various  nations,  and  this  is  a  very 
significant  fact,  especially  as  modern  cytology  is  a  much 
debated  subject.  If  it  is  possible  in  any  branch  of  knowledge 
to  speak  of  a  sententia  communis  doctorum,  we  may  regard 

1  Jena,  1905,  p.  50  :    '  The  cell  is  a  little  mass  of  protoplasm  containing 
one  or  more  nuclei.' 

2  Precis  d'Histologie,  Paris,  1900,  p.  26  :    '  La  cellule  est  essentiellement 
une  petite  masse  de  protoplasma  avec  un  noyau.' 

3  1906,  p.  27  :    '  The  nucleus  is  just  as  essential  to  the  existence  of  a  cell 
as  is  the  protoplasm.'     Cf.  also  the  more  detailed  account  given  by  0.  Hertwig 
in  the  third  chapter  of  the  same  work. 


THE  CELL  49 

the  definition  of  a  cell  as  such  in  a  very  conspicuous 

I  must  acknowledge,  however,  that  this  unanimity  exists 
among  zoologists  and  histologists  more  than  among  botanists.1 

In  many  of  the  smallest  forms  of  plant  life,  especially  in 
many  bacteria,  the  presence  of  a  true,  clearly  differentiated 
nucleus  has  not  yet  been  established.3  I  use  the  words  '  true, 
clearly  differentiated  nucleus  '  advisedly,  for  cytologists  are 
more  and  more  adopting  the  opinion  that  even  in  those  micro- 
organisms previously  regarded  as  devoid  of  nucleus  the 
nuclear  substance  is  present,  though  divided  into  smaller 
particles,  which  E.  Hertwig  has  designated  chromidia?  This 
opinion  gains  support  from  the  discovery  of  a  true  nucleus 
existing  at  a  definite  stage  in  the  formation  of  the  spores  of 
the  Bacillus  Butschlii.41 

We  shall  have  to  return  later  on  (Chapter  VII)  to  the  most 
recent  investigations  made  by  biologists  on  the  subject  of  the 
absence  of  nucleus  in  these  extremely  small  forms  of  life.  For 
the  present  it  is  enough  to  say  that  the  idea  of  a  living  cell 
involves  that  of  a  nucleus,  either  as  a  whole  or  in  parts,  but 
the  chromatophores  that  exist  in  most  plant  cells  besides  the 
cytoplasm  and  the  nucleus  are  certainly  not  essential  to  the 
existence  of  the  cell,  for  they  are  absent  in  Bacteria  and  fungi, 
and  in  all  animal  cells.5 

Let  us  now  proceed  to  study  the  structure  of  a  cell  more 
in  detail. 

In  shape  and  size  cells  vary  greatly.  The  normal  shape  of 
a  free  cell,  not  united  with  others  of  the  same  kind  to  form  a 
tissue,  is  spherical,  but  even  the  unicellular  plants  and  animals 
are  seldom  quite  round,  and  cells  united  to  form  tissues  still 
less  often  approach  a  spherical  shape  ;  they  are  rounded,  or 
oval,  or  cylindrical,  or  cubical,  or  pentagonal,  or  hexagonal ; 

1  Cf.  Lehrbuch  der  Botanik  fur  Hochschulen  by  Strasburger,  Noll,  Schenk  and 
Karsten,  6th  edit,,  Jena,  1904,  pp.  46-7,  270,  274,  where  it  is  stated  that  the 
presence  of  a  nucleus  in  the  lowest  plants  (Cyanophyceae  and  Bacteria)  is 
still  uncertain.     (English  translation,  3rd  edit.  1908,  pp.  53  and  332.) 

2  Cf.  J.  Reinke,  Einleitung  in  die  theoretische  Biologie,  1901,  pp.  256,  &c. 

3  R.  Hertwig,  '  Die  Protozoen  und  die  Zellentheorie  '  (Archiv  fur  Protisten- 
kunde,  I,  1902,  pp.  1-40). 

4  Fr.  Schaudinn,  '  Beitrage  zur  Kenntnis  der  Bakterien  und  verwandter 
Organismen,'     I.  Bacillus   Biitschlii,   n.    sp.     (Archiv  fur   Protistenkunde,   I, 
pp.  306,  &c.). 

3  Cf.  Strasburger,  &c.,  pp.  46,  47  (Eng.  trans,  p.  53). 



sometimes  they  are  of  almost  the  same  thickness  in  all  three 
dimensions,  at  other  times  they  are  flattened  out  like  those  of 
the  pavement  epithelium  (fig.  2d),  or  extraordinarily  long,  like 


FIG.  2. 

Magnified  230  times     [Zeiss  D,  Ocul.  2]. 

All  the  figures  have  been  prepared  with  the  camera  lucida  from  series  of 

KEY  TO  FIG.  2. 

a  =  Giant  cell  containing  two  nuclei  from  the  abdominal  fat-body  of  a 

physogastric  specimen  of  Termitoxenia  Heimi  Wasm. 
zk,  zk  =  nuclei. 

b  =  young  egg  of  Termitoxenia  Heimi  Wasm.  The  egg -cell  is  still  enclosed 
within  the  follicular  epithelium  of  the  ovary.  (From  a  sagittal 
section  of  a  physogastric  specimen  of  Termitoxenia  Heimi. ) 

ep  —  epithelial  cells  of  the  one-layered  follicle. 

zk  =  nuclei  of  the  epithelial  cells. 

kb  —  germinal  vesicle  of  the  egg. 

kf  =  nucleolus  of  the  germinal  vesicle. 

dd  =  vitelline  spherules. 

nk  =  remains  of  the  nucleus  of  a  nutritive  cell,  the  material  of  which  has 

served  to  form  the  yolk. 

c  =  three  unicellular  muscular  fibres  from  the  cutaneous  muscular  apparatus 
of  the  abdomen  of  a  stenogastric  specimen  of  Termitoxenia  (Termi- 
tomyia)  mirabilis  Wasm. 

zk  =  nucleus. 

d  =  two  epithelial  cells  from  the  hypodermis  of  the  abdomen  of  a  steno- 
gastric specimen  of  Termitoxenia  Heimi. 

zk  =  nucleus. 

the  spindle-shaped  cells  of  the  smooth  muscular  fibres,  and 
the  still  more  slender  cells  that  form  the  transversely  striated 
muscular  fibres  (fig.  2c). 

THE  CELL  51 

As  a  rule,  the  cells  that  make  up  tissues  have  no  prolonga- 
tions, but  in  making  this  statement  I  am  not  challenging 
Heitzmann's  discovery  (1873)  of  protoplasmic  cell-bridges.1 
Many  cells,  however,  possess  long  offshoots,  which  give  them 
a  ramified  appearance  ;  this  is  particularly  the  case  with  nerve- 
cells,  and  is  closely  connected  with  their  telegraphic  functions. 

The  shape  of  the  nucleus  varies  less  than  that  of  the  cell,2 
it  is  mostly  round  or  oval,  although  other  shapes  not  in- 
frequently occur.  Very  remarkable  are  the  branching  nuclei 
of  the  Malpighian  tubes  in  certain  caterpillars,  and  the  nuclei 
resembling  a  string  of  beads  in  some  unicellular  Stentors. 

In  speaking  of  the  size  of  a  cell,  we  must  have  a  standard 
by  which  to  measure  it.  In  this  respect  little  cells  resemble 
so-called  tall  men  ;  we  cannot  measure  either  by  any  usual 
method,  an  old-fashioned  foot-rule  and  a  modern  metre 
measure  are  equally  out  of  place.  Cells  have  to  be  measured 
under  the  microscope,  and  the  following  method  is  the  simplest. 
The  number  of  times  that  the  object  is  magnified  is  carefully 
noted,  and  a  sketch  of  the  cell  is  made  on  paper  by  means  of  a 
camera  lucida.  This  sketch  is  then  measured  with  a  very 
exact  millimetre  measure,  and  the  number  thus  obtained  is 
divided  by  that  of  the  magnifying  power.  For  instance,  if 
a  cell,  magnified  230  times,  measures  23  mm.,  its  real  magni- 
tude is  O'l  mm.  This  would  be  a  giant  cell  if  it  belonged  to 
animal  tissue.  Such  giant  cells  as  this  (cf.  fig.  2a)  compose 
the  abdominal  fat-body  of  the  Termitoxenia,  a  variety  of 
Diptera  living  among  termites,  as  we  have  already  seen  (pp.  37, 
&c.).  Most  cells  in  animal  tissues  are  dwarfs  in  comparison, 
and  dwarfs  among  dwarfs  are  the  average  blood  corpuscles, 
especially  of  insects,  and  the  spermatozoa  of  most  animals. 
Therefore,  as  a  constant  unit  for  microscopical  measurement 
of  cells,  the  thousandth  part  of  a  millimetre  has  been  adopted, 
which  is  known  as  a  micromillimetre  or  micro,  and  is  designated 
by  the  letter  JJL.  The  giant  cells  of  the  Termitoxenia' s  fat- 
body  have  a  diameter  of  lOOyu,.  Cells  of  ID//,  (e.g.  figs.  2d 

1  A  further  account  of  these  protoplasmic  cell-bridges  will  be  found  in 
Wilson,  The  Cell,  pp.  56,  60,  where  there  is  a  careful  discussion  of  the  evidence 
for  their  existence  among  very  various  kinds  of  plant  and  animal  cells.     See 
also  0.  Hertwig's  Allgemeine  Biologie,  pp.  400,  &c. 

2  For  the  shape,  size,  and  number  of  nuclei,  see  0.  Hertwig,  Allgemeine 
Biologie,  pp.  28,  &c. 

E  2 


and  2b,  &c.)  are  of  medium  size,  so  the  former  may  well  be 
called  gigantic. 

But  there  are  some  animal  cells  far  larger  than  these,  viz. 
the  egg-cells.  These  are  the  largest  in  the  animal  kingdom.1 

The  ripe  egg-cell  of  a  diminutive  insect  such  as  the  Ter- 
mitoxenia,  barely  2  mm.  in  length,  measures  almost  1  mm.,  i.e. 
it  is  half  as  long  as  the  creature's  whole  body.  The  eggs  of 
this  fly  are  reckoned,  therefore,  among  the  relatively  largest  in 
the  entire  animal  kingdom ;  the  absolutely  largest  occur 
among  birds  ;  it  is  in  fact  possible  to  use  a  yard  measure  to 
ascertain  the  size  of  the  eggs  of  the  ostrich  or  moa.  A  bird's 
egg  before  fecundation  consists  of  one  huge  cell,  but  to  the 
egg-cell  belong  in  this  case  not  only  the  germinal  vesicle, 
which  represents  the  nucleus  of  the  protoplasmic  part  or 
formative  yolk  of  the  egg-cell,  but  also  a  quantity  of  nutritive 
yolk  or  deuteroplasm,3  which  is  really  the  yolk  of  the  bird's 
egg.  The  white  of  the  egg  and  the  shell  appear  only  after 
fecundation,  and  are  outer  coverings,  and  not  parts  at  all  of 
the  egg-cell.  Animal  egg-cells  owe  their  conspicuous  size  to 
the  presence  of  deuteroplasm  or  nutritive  yolk,  which  is  found 
in  the  eggs  of  all  creatures  that  are  oviparous  and  not  vivi- 
parous. In  the  case  of  the  former  a  considerable  quantity  of 
nutritious  matter  must  be  stored  up  in  the  egg  itself,  in  order 
that  the  embryo  may  develop.  My  readers  must  not,  how- 
ever, fancy  that,  when  they  see  a  new-laid  hen's  egg,  they  have 
only  one  huge  egg-cell  before  them  ;  for,  quite  apart  from  the 
above-mentioned  exterior  coverings,  which  grow  before  the 
egg  is  laid,  the  egg  itself  is  already  fertilised,  its  germinal 
vesicle  has  become  a  germinal  disc,  i.e.  a  still  very  diminutive 
embryo  chick,  consisting  of  numerous  segmentation  cells,  and 
the  huge  egg  serves  as  its  lodging  and  store-room  during  its 
further  development. 

In  order  to  illustrate  the  various  shapes  and  sizes  of  the 
cell  by  examples,  I  have  reproduced  some  cells  of  Termitoxenia 
on  p.  50.  To  the  explanations  already  given  I  may  add  that, 

1  Very  large  cells  constitute  the  plasmodia  of  the  Mycetozoa,  which  are 
also  reckoned  among  the  lower  orders  of  plants  and  called  Myxomycetes, 
whilst  by  others  again  they  are  classed  with  the  Protozoa.     Cf.  R.  Hertwig, 
Lehrbuchder  Zoologie,  7th  edit.,  1905,  pp.  49  and  168  (Eng.  trans,  pp.  60,  61, 198). 

2  E.  van  Beneden  called  the  nutritive  yolk    'deutoplasm,'  to  contrast   it 
with  protoplasm  ;   '  deuteroplasm '  is  a  more  correct  form  of  the  word. 

THE  CELL  53 

with  a  view  to  economising  space,  I  chose  for  Fig.  2b  not  a 
ripe  and  fully  developed  egg-cell,  but  a  young  cell,  still 
surrounded  by  a  thick  follicle  of  epithelial  tissue,  and  having 
at  its  lower  end  the  remains  of  an  incompletely  consumed 
nutritive  cell.  As  the  latter  is  already  incorporated  with  the 
substance  of  the  egg,  the  young  cell  (without  the  epithelium) 
measures  135//,  in  length  and  95//,  in  breadth.  A  ripe  egg- 
cell  of  the  same  kind  of  Termitoxenia  would,  if  drawn  on  the 
same  scale  (magnified  230  diameters),*  occupy  a  space  of 
2  dm.,  and  cover  a  whole  page  of  this  book. 

Some  plant  cells  are  also  very  large  ;  for  instance,  there 
are  bast-cells  2  dm.  in  length  and  of  considerable  breadth. 
Among  the  lower  plants  too,  such  as  the  Caulerpa  (one  of  the 
Algae),  there  are  cells  several  decimetres  in  length;  in  fact, 
according  to  J.  Reinke  and  other  botanists,  the  whole  plant  with 
its  root,  stem,  and  leaves  consists  of  one  cell  with  many  nuclei.1 

The  dwarfs  among  plant  cells  are  many  of  the  Bacteria, 
which  have  a  longitudinal  diameter  of  not  quite  lyu,  (y-^-^mm.). 
The  petal  of  a  violet  consists  of  about  50,000  cells  which  are 
comparatively  large. 

By  far  the  greater  number  of  cells  have  but  one  nucleus, 
and  if  they  are  found  to  contain  more  than  one,  it  is  generally 
because  the  process  of  cell-multiplication  by  division  is  just 
beginning.  There  are,  however,  some  cells  that  always 
contain  several  nuclei ;  such  are,  for  instance,  those  in  the 
marrow  of  vertebrates,  and  partly  also  those  known  as  syncytia 
in  the  adipose  tissue  of  insects  and  other  Arthropods.3 

In  his  classical  and  suggestive  work  on  cell-division  among 
the  Arthropods,3  Carnoy  expresses  the  opinion  that  these  are 
all  multinuclear  giant  cells,  not  masses  of  cells  formed  by  the 
fusion  of  others.  This  view  cannot  be  adopted  without  reserva- 
tion, as  there  are  undoubtedly  cases  in  which  syncytia  arise 
from  a  gradual  breaking  down  of  the  cell- walls.  This  takes 
place,  for  instance,  in  Termitomyia,  a  sub-genus  of  Termito- 
xenia. In  the  sub-genus  Termitoxenia  (in  the  narrower  sense) 

1  See  Reinke,  Einleitung  in  die  iheoretische  Biologie,  p.  213,  and  his  Mono- 
graphie  der  Gattung  Caulerpa.     See  also  Frank,  Synopsis  der  Pftanzenkunde, 
III,  Hanover,  1886,  §  890;  van  Tieghem,  Traite  de  Botanique  (1891),  pp.  9,  10. 

2  On  the  subject  of  syncytia  or  cell-fusions  see  also  0.  Hertwig,  Allgemeine 
Biologie  (1906),  pp.  378-381. 

3  '  La  Cytodierese  chez  les  Arthropodes '  (La  Cellule,  I, 1885,  n.  2,  p.  235,  &c.). 


these  adipose  cells  are  very  large,  but  they  are  distinct  one 
from  the  other,  though  in  full-grown  physogastric  specimens, 
in  which  no  further  cell- division  occurs,  there  are  frequently 
two  nuclei  (cf.  fig.  2a)  instead  of  one.  According  to  Weismann1 
multinuclear  cells  occur  also  in  the  festooned  columns  of 
cells  found  in  the  larvae  of  flies.  I  have  myself  found  cells 
with  two  or  more  nuclei  in  the  halteres  of  Termitoxenia,  and 
Bolles  Lee  discovered  them  before  me  in  those  of  common 
Diptera.2  In  many  of  the  lower  orders  of  plants,  such  as  the 
Thallophyta,  cells  containing  several  or  even  many  nuclei  are 
of  frequent  occurrence,  and  among  the  Siphonaceae,  a 
subdivision  of  the  Algae,  there  are  plants  (C  aider  pa,  Vaucheria, 
&c.),  which  consist  of  one  huge  multinuclear  cell,  as  has  been 
already  stated. 

Just  as  in  the  tissues  of  living  organisms  there  may  be,  and 
actually  are,  cells  which  contain  several  nuclei,  but  still  do 
not  divide  into  more  cells,  so,  in  the  lowest  forms  of  animal 
life,  tke  Protozoa,  there  are  unicellular  organisms  containing 
two  or  more  nuclei,  but  not  forced  on  that  account  to  split 
up  into  several  individuals. 

The  reader  must,  however,  carefully  distinguish  the  multi- 
nuclear  cells  just  mentioned,  from  others  which  contain  beside 
or  in  the  true  nucleus  one  or  more  little  round  bodies  known 
as  nucleoli.  The  founders  of  cytology,  Schleiden  and  Schwann, 
noticed  these  bodies  and  regarded  them  as  having  some 
essential  importance  in  the  structure  of  the  cell.  This  opinion 
has  proved  to  be  erroneous,  and  most  nucleoli  seem  to  be  merely 
differentiations  of  the  ordinary  substance  of  the  nucleus.  For 
this  reason  I  have  purposely  refrained  from  referring  to  them 
until  now,  when  we  are  concerned  with  the  more  detailed 
morphology  of  the  cell. 


In  an  account  of  the  origin  of  modern  cytology,  Gustav 
Schlater  writes  as  follows  :  3  '  The  cell  is  a  little  mass  of  proto- 
plasm, endowed  with  all  the  properties  of  life.  This  was  the 

1  Die  Entwicklung  der  Dipteren,  Leipzig,  1864,  p.  132  and  Plate  8,  fig.  10. 

2  '  Les  balanciers  des  Dipteres  '    (Eecueil  Zoolog.  Suisse,  II    (1885),  389 
et  pi.  XII,  fig.  18). 

3  G.  Schlater,  '  Der  gegenwartige  Stand  der  Zellenlehre  '  (Biolog.  Zentral- 
blatt,  XIX,  1899,  Nos.  20-24,  p.  667). 


definition  given  by  Max  Schultze,  and  at  the  time  our  idea  of 
a  cell  seemed  to  have  reached  its  full  development.  Thence- 
forth, we  had  only  to  submit  cells  to  examination  from  many 
points  of  view,  and  the  representatives  of  every  branch  of 
biology  did  in  fact  turn  their  attention  to  the  cell.  The  word 
"Protoplasm"  was  ever  on  their  lips,  and  the  number  of  works 
devoted  to  the  examination  of  the  structure  and  life  of  this 
elementary  unit  in  living  substance  is  so  great  that  it  would 
be  quite  impossible  for  anyone  to  read  them  all.  This 
examination  has  proved  very  fertile  in  results  ;  every  step  has 
supplied  fresh  evidence  supporting  the  general  biological 
importance  of  the  cell-theory  ;  every  book  written  has  proved 
that  we  must  start  from  the  cell  in  order  to  extend  our  know- 
ledge of  nature.  The  reputation  of  the  cell  increased  ;  it 
revealed  itself  as  more  and  more  complex  in  its  formation. 
Within  it,  in  this  little  mass  or  drop  of  living  substance,  modern 
research  has  discovered  a  complicated  structure,  and  more 
and  more  details  of  this  structure,  and  each  day  adds  to  the 
interest  taken  by  men  of  science  in  the  whole  complicated 
vital  processes  that  go  on  in  the  small  compass  of  the  cell.' 

The  interesting  question  arises  here  :  Are  we  to  consider 
the  cell  simple  or  complex  ?  Is  it  the  ultimate  biological 
unit  in  the  structure  of  organisms,  or  is  it  itself  a  diminutive 
organism  made  up  of  subordinate  units  ?  This  is  a  weighty 
question,  having  an  important  bearing  on  the  problem  of  life, 
and  students  are  apt  to  overlook  its  twofold  character.  In 
order  to  emphasise  it,  let  us  divide  the  question  into  two,  and 
ask  :  (1)  Is  the  cell  morphologically  simple  ?  (2)  Is  it  the 
ultimate  biological  unit  of  organic  life,  or  is  it  an  aggregation 
of  lower  elementary  units  ?  It  is  possible  to  deny  the  simplicity 
of  the  cell  and  at  the  same  time  to  affirm  its  unity,  for,  according 
to  the  unchanging  laws  of  thought  which  are  still  binding  upon 
the  Homo  sapiens  of  the  twentieth  century,  simplicity  and 
unity  are  two  quite  different  ideas.  Modern  research  will 
never  attain  to  assured  philosophical  results  regarding  the 
nature  of  life,  if  it  confuses  unity  and  simplicity.  Let  us  try 
to  give  to  both  questions  an  answer  based  upon  facts.1 

1  Cf.  0.  Hertwig,  Allgemeine  Biologic,  1906,  chapters  ii  and  iii ;  Wilson, 
The  Cell,  1902  ;  Yves  Delage,  La  structure  du  protoplasma  et  Us  theories  sur 
rheredite,  Paris,  1895. 


Is  the  cell  simple  ?  No,  it  is  not  simple,  but  extremely 
complex  in  many  cases,  a  true  microcosm.  It  consists  of  a 
number  of  parts  that  differ  morphologically,  chemically,1  and 
physiologically,  and  yet  on  their  harmonious  connexion 
depends  the  biological  unity  of  the  vital  process  of  the  cell. 
Although  all  parts  of  the  cell  participate  more  or  less  in  its 
vital  activities,  still  the  nucleus  is  of  chief  importance  in  the 
principal  processes.2 

Such  are  briefly  the  results  of  the  most  recent  investigations 
of  cytology,  and  we  have  now  to  consider  them  more  in  detail.3 

The  two  chief  morphological  constituents  of  the  cell  are  the 
cell-body  and  the  nucleus,  and  this  has  been  universally 
acknowledged  ever  since  Leeuwenhoek  discovered  the  nucleus 
(see  p.  31).  At  the  present  time  everyone  regards  them  as 
essential  to  the  cell,  whilst  the  membranous  covering  of  the 
cell  and  the  nucleoli  within  the  nucleus  are  not  essential.4  In 
1882  Strasburger  suggested  the  name  cytoplasm  to  designate  the 
protoplasm  of  the  cell-body,  and  his  suggestion  has  generally 
been  adopted.5 

It  was  originally  regarded  as  absolutely  homogeneous,  but 
after  Dujardin's  study  of  it  (1835)  little  granules  were  noticed 
in  it,  and  further  examination  revealed  a  structure  variously 
described  as  filar,  reticular,  or  alveolar.  There  are  many 
modern  theories  regarding  the  structure  of  cytoplasm.  All 
students,  with  the  exception  of  those  mentioned  first,  agree 
in  recognising  in  the  protoplasm  of  the  cell-body  two  distinct 
substances,  one  being  transparent  and  forming  the  foundation  of 

1  The  chemical  constituents  of  protoplasm  and  the  morphological  variety 
of  the  parts  of  the  cell  are  not  discussed  here  in  detail,  because  very  little 
is  as  yet  known  with  certainty  about  them.     (Cf.  Chapter  II,  p.  33.)     How 
complicated  the  chemical  composition  of  the  nucleus  is  may  be  seen  on  reference 
to  Dr.  Hans  Malfatti's  work,    '  Zur  Chemie  des  Zellkerns '   (BericMe  des  natur- 
wissenschaftlich-medizinischen  Vereins,  Innsbruck,  XX,   1891-2). 

2  This  fact  is  acknowledged  even  by  those  who,  like  J.  Reinize,  regard  it 
as  not  essential  to  differentiate  the  nucleus  as  a  distinct  morphological  forma- 
tion.    (See  Reinke's  Einhitung  in  die  theoretische  Biologie,  1901,  p.  256.) 

3  An  excellent  account  of  the  morphology  of  cells  and  of  the  various 
theories  regarding  the  structure  of    the  cell-body  and  the  nucleus  will  be 
found  in  Wilson's  The  Cell,  pp.  19-62. 

4  The  subject  of  the  centrosomes  will  be  reserved  for  discussion  in  Chapter 
V.     See  0.  Hertwig,  Allgemeine  Biologie,  pp.  45-49. 

5  0.  Hertwig  prefers  to  retain  the  older  meaning  of  the  word  protoplasm, 
in  which  it  was  originally  used  by  von  Mohl,  Max  Schultze  and  Leydig,  to 
designate  the  substance  of  the  cell-body  as  distinct  from  the  nucleus.     Stras- 
burger's  cytoplasm  is  thus  identical  with  the  protoplasm  of  these    earlier 


the  cell  (hyaloplasm,  as  Ley  dig  calls  it),  and  the  other  granular, 
consisting  of  microsomes,  which  form  the  framework  of  the  filar, 
reticular,  or  alveolar  structure  (spongioplasm,  as  Leydig  calls 
it).  The  former  is  also  very  suitably  called  cytoplasm,  and  the 
latter  cytomitom,  but  a  great  number  of  names  have  been  given 
to  both,1  names  calculated  to  astound  any  ancient  Hellene  who 
heard  the  modern  derivatives  coined  from  the  wealth  of  old 
Greek  words. 

Those  who  believe  cytoplasm  to  be  homogeneous  do  not 
recognise  the  presence  in  the  living  cell  of  two  morphologically 
distinct  substances,  but  they  regard  the  granules  and  threads 
and  meshes  of  the  so-called  cell-framework  as  merely  artificial 
products,  resulting  from  the  chemical  reactions  and  the  use 
of  stains  for  microscopical  purposes. 

There  are,  however,  good  reasons  why  this  theory  does 
not  find  many  supporters  at  the  present  day,2  for  recent  micro- 
scopical research  has  revealed  in  the  living  cell  a  structure, 
which  is  not  produced  by  the  processes  of  fixing  and  stain- 
ing, but  is  only  rendered  visible  by  means  of  them.  This  is 
especially  true  of  the  filar  structure  of  spongioplasm,  which  is 
practically  identical  with  the  reticular  structure  or  frame- 
work. It  was  discovered  first  by  Karl  Frommann  in  1875, 
but  Flemming  recognised  it  as  filar,3  and  his  observations 
have  been  confirmed  by  those  of  many  other  scientists, 
such  as  Klein,  Leydig,  E.  van  Beneden,  Carnoy,  Heidenhain, 
Zimmermann,  &c.,  and  are  now  regarded  as  of  unquestioned 
accuracy.  It  is  of  secondary  importance  to  decide  whether,  as 
Flemming  thinks,  the  protoplasmic  threads  are  of  greater 
significance,  or,  in  agreement  with  Klein,  Carnoy,  &c.,  we 
should  lay  stress  particularly  on  the  network  formed  by  these 

Butschli's  alveolar  theory  represents  another  view  of  the 
structure  of  the  cell.  According  to  it  the  protoplasm  of  the 

1  See  Biitschli,   '  tiber  die    Struktur    des    Protoplasmas,'    19    (Verhandl. 
der  deutschen  Zoolog.  Gesellsch.,  1891,  pp.  14-29). 

2  A.  Fischer,  whose  theory  regarding  the  polymorphic  character  of  proto- 
plasm will  be  discussed  later  on,  must  not  be  reckoned  among  those  who 
uphold  the  homogeneity  of  protoplasm. 

3  See  W.  Flemming,  '  Uber  den  gegenwartigen  Stand  unserer  Kenntnisse 
und  Anschauungen  von    den   Zcllstrukturen,'  a  paper  read  at    the  opening 
of  the  thirteenth  meeting  of  the  Anatomical  Society  at  Tubingen  on  May  22, 
1899  (Naturwissenschajtliche  Rundschau,  XIV,  1899,  Nos.  35  and  36). 


cell  has  a  structure  resembling  honeycomb  or  foam,  due  to 
the  mechanical  mixture  of  the  various  fluid  constituents  of 
protoplasm.  That  suspended  in  the  fluid  hyaloplasm  there 
are  often  vacuoles,  filled  with  another  kind  of  fluid,  is  a  fact  not 
questioned  even  by  the  opponents  of  this  theory,  but  they 
deny  that  the  minute  structure  of  the  protoplasm  depends 
merely  upon  the  presence  of  these  vacuoles  ;  for,  whereas 
spongioplasm,  treated  according  to  Biitschli's  methods,  ap- 
peared to  reveal  an  alveolar  structure,  closer  examination  has 
shown  that  a  reticular  structure  really  underlies  it.  The  chief 
evidence  brought  forward  by  Biitschli  in  support  of  his  alveolar 
theory  is  derived  from  artificial  mixtures  of  various  fluids, 
which  bear  a  superficial  resemblance  to  cell-structures,  but 
cannot  of  themselves  prove  anything  about  the  real  structure 
of  the  cell. 

I  have  no  wish,  however,  to  condemn  Biitschli's  alveolar 
theory,  for  we  ought,  in  speaking  of  it,  to  distinguish  between 
his  view  of  the  honeycomb  structure  of  the  cell,  and  his  explana- 
tion of  that  structure  by  assuming  a  mechanical  mixture  of 
various  fluids.  The  latter  hypothesis  is  extremely  doubtful, 
and  has  been  thoroughly  discussed  by  Oskar  Hertwig  in  his 
'  Allgemeine  Biologie  '  (p.  23).  On  the  other  hand,  Biitschli's 
theory  of  the  alveolavr  structure  of  many  cells  has  been 
strengthened  by  recent  research.  In  very  thin  microscopical 
sections  very  highly  magnified,  what  appears  as  a  network 
seems  in  fact  often  to  be  only  a  section'of  a  framework  consisting 
not  of  meshes  but  of  closed  chambers  ;  and,  if  this  is  true,  in 
these  particular  cells  the  protoplasm  has  really  not  a  reticular 
but  an  alveolar  structure.  In  my  series  of  sections  of  the 
large  gland-cells  in  the  wing-covers  of  a  termitophile  beetle 
(Chaetopisfhes  Heimi)  I  have  occasionally  perceived  a  distinctly 
alveolar  structure  of  the  spongioplasm.1  It  seems,  therefore, 
that  the  alveolar  theory  may  stand  beside  the  reticular  theory, 
although  latterly  it  has  been  attacked  by  those  who  are 
inclined  to  regard  the  alveoli  seen  under  the  microscope  as  an 
artificial  product,  or  as  a  pathological  vacuolisation  of  the 

1  Of.  '  Zur  naheren  Kenntnis  des  echten  Gastverhaltnisses  bei  den  Ameisen- 
und  Termitengasten  '  (Biolog.  Zentralblatt,  XXIII,  1903,  Nos.  2-8,  p.  269). 

2  Cf.  A.  Degen,  '  Untersuchungen  iiber  die  kontraktile  Vakuole  nnd  Waben- 
struktur  des  Protoplasmas  '  (Botanische  Zeitung,  1905,  Part  I,  pp.  163-225). 


Less  satisfactory  than  Biitschli's  alveolar  theory  is 
Altmann's  granular  theory,1  which  is  based  upon  the  granular 
structure  of  protoplasm.  If  Altmann  merely  asserted  that 
numerous  granules,  now  generally  termed  microsomes,  are 
embedded  in  the  transparent  hyaloplasm  of  the  cell,  there 
would  be  no  objection  to  his  theory,  for  it  would  rest  on  actual 
observations.  But  he  goes  on  to  deny  the  fibrillar  or  reticular 
structure  of  the  spongioplasm,  and  thinks  that  it  may  be 
explained  as  a  close  series  of  granules.  Flemming,  on  the 
other  hand,  rightly  points  out  that  the  microsomes  are  often 
arranged  like  beads  on  the  reticular'  framework,  but  do  not 
actually  form  that  framework.  Moreover,  a  large  proportion 
of  Altmann's  famous  granules  have  been  proved  not  to  be 
microsomes  at  all,  but  merely  artificial  products  accidentally 
resulting  from  chemical  reaction  ;  in  fact,  they  are  metaplasmic 
bodies  and  consist  of  protoplasm  and  foreign  substances 
embedded  in  it,  and  were  mistaken  by  Altmann  for  his  granules, 
and  the  scientific  value  of  his  theory  is  greatly  diminished  in 
consequence.  Its  chief  defect,  however,  is  that  it  regards  the 
granules  contained  in  protoplasm  as  alone  forming  its  essential 
active  basis,  and  that  it  boldly  accepts  them  as  elementary 
organisms  out  of  which  the  cell,  as  a  secondary  formation,  is 
composed.  This  view  is  devoid  of  all  real  foundation  in  facts, 
and  has  been  rejected  by  most  scientists.  We  shall  have  to 
refer  to  it  again  later,  in  discussing  the  unity  of  the  cell. 

There  is  great  diversity  of  opinion  as  to  the  relative  im- 
portance of  the  two  morphologically  distinct  constituents  of 
the  cell-body,  viz.  hyaloplasm  (cytoplasm)  and  spongioplasm 
(cytomitom).  Heitzmann,  van  Beneden,  Eeinke,  Carnoy, 
Ballowitz  and  others  agree  in  thinking  the  latter,  which  forms 
the  framework  of  the  cell,  its  really  living,  moving  and  con- 
tractile element,  whereas  others,  and  especially  Leyden, 
ascribe  these  qualities  to  the  former,  and  regard  the  hyaloplasm 
as  the  living  substance.  As  Flemming  saw,  these  two  opinions 
ought  probably  to  be  united,  for,  as  no  living  cell  contains 
hyaloplasm  exclusively  or  spongioplasm  exclusively,  both 
must  be  considered  essential  constituents  of  protoplasm, 
although  most  scientists  agree  with  Flemming  in  assigning 

1  Cf.    Richard  Altmann,  Die  Elementarorganismen  und   ihre   Beziehungen 
zu  den  Zellen,  1894. 


greater  importance  to  spongioplasm  than  to  hyaloplasm.  It 
is  obvious  that  for  the  present  we  must  be  content  to  accept 
hypotheses  of  various  degrees  of  probability,  and  these  various 
theories  regarding  the  more  minute  structure  of  the  cell  are  all 
more  or  less  of  a  hypothetical  character. 

Quite  recently,  in  1895-6,  another  theory  as  to  the 
structure  of  the  cell  has  been  brought  forward  by  Friedrich 
Beinke  and  elaborated  by  Wilhelm  Waldeyer,  and  Gustav 
Schlater  calls  it  the  newest  achievement  of  modern  research 
into  the  morphology  of  the  cell.1  This  theory  attempts  to 
reconcile  the  various  views  as  to  the  structure  of  protoplasm. 
According  to  it,  in  the  homogeneous  ground-substance  of  the 
cell  (i.e.  in  the  cytoplasm,  as  other  writers  call  it)  there  is 
embedded  a  reticular  framework  (cytomitom) ;  the  formation 
of  the  latter  varies,  but  in  the  main  it  is  alveolar  and  in  its 
walls  lie  very  small  granules  (microsomes) ,  which  in  certain 
cases  are  aggregated,  so  as  to  form  filaments  and  network. 
The  chief  framework  of 'the  cell  owes  its  alveolar  structure 
to  the  larger  vacuoles  and  granules  which  it  contains.  Keinke- 
Waldeyer's  theory  thus  harmonises  the  views  of  other  scientists, 
and  we  may  regard  it  as  summing  up  all  that  was  known  of  the 
structure  of  the  cell  in  the  year  1900  ;  there  is,  however,  one 
drawback  to  it  theoretically,  for  it  lays  too  little  stress  upon 
an  essential  element,  viz.  the  meshwork  or  alveolar  structure 
of  the  cell-framework,  with  the  rows  of  microsomes  arranged 
along  it,  and  it  lays  comparatively  too  much  stress  upon  an 
unessential  element,  viz.  the  vacuoles  and  larger  granules 
which  the  cell  contains. 


Hitherto  we  have  discussed  only  the  details  of  the  cell-body, 
now  we  must  consider  the  structure  of  the  nucleus.  Here 
again  we  find  two  chief  substances,  which,  however,  differ 
morphologically,  physiologically,  and  chemically  far  more 
from  one  another  than  do  the  spongioplasm  and  the  hyaloplasm 
of  the  cell-body.  It  is  often  possible  to  discover  in  the  nucleus 
not  only  two,  but  three  or  four  protein  substances  differing 
under  chemical  and  microscopical  examination.  The  nucleus  is 

1  Biolog.  Zentralblatt,  XIX,  1899,  No.  20,  p.  676. 


therefore,  as  0.  Hertwig  rightly  remarks,  a  very  complex l 
formation,  so  far  as  its  constituents  are  concerned.  According 
to  their  behaviour  when  stains  are  applied  to  them  to  facilitate 
their  microscopical  examination,  the  two  chief  substances  in 
the  nucleus  have  been  called  chromatin  and  achromatin ; 
according  to  their  chemical  properties  they  are  called  nuclein 
and  linin  respectively.  Chromatin  or  nuclein  takes  a  brilliant 
colour  when  treated  with  carmine,  haematoxylin,  &c.,  whereas 
achromatin  or  linin  is  either  not  stained  at  all  or  takes  a  colour 
only  under  special  circumstances.  Achromatin  resembles 
in  structure  the  protoplasm  of  the  cell-body,  for  it  contains 
a  fluid  known  as  karyoplasm,  and  a  fibrillar  or  reticular  or 
alveolar  framework  known  as  karyomitom.  These  are  analogous 
to  the  cytoplasm  and  cytomitom  of  the  cell-body.  Large  nuclei 
are  bounded  on  the  outside  by  a  peculiar  nuclear  membrane. 

Chromatin  has  been  mentioned  as  one  of  the  chief  substances 
in  the  nucleus  ;  the  parts  that  are  readily  stained  are  formed 
of  it,  and  it  is  composed  of  nuclein.2 

Closely  connected  with  it,  though  differing  chemically 
both  from  chromatin  and  from  achromatin  or  linin,  is  another 
substance,  less  readily  stained,  known  as  plastin  or  paranuclein. 
Nuclein  and  plastin  together  form  the  chromatin  nucleoli,  the 
chromatin  nuclear  framework,  or  the  chromatin  skein-like 
nuclear  filaments  ;  these  are  only  different  names  for  the 
different  forms  assumed  by  the  nuclein-plastin  elements  in  the 

With  regard  to  the  relation  in  which  they  stand  to  the 
achromatic  nuclear  framework,  many  theories  have  been  pro- 
pounded by  Memming,  Carnoy  and  others,  but  we  cannot 
discuss  them  in  detail  now.  For  the  present  let  it  suffice  to 
say  that  two  distinct  kinds  of  nucleoli  have  been  discovered,  the 
one  kind  very  readily  stained,  the  other  less  so,  but  both  con- 
sisting of  combinations  in  different  proportions  of  nuclein 
and  paranuclein,  whilst  on  the  other  hand  the  true  nucleoli  or 
plasmosomes  are  not  susceptible  to  any  stain,  consist  only  of 
paranuclein  (pyrenin),  and  form  more  or  less  transparent 

1  Allgemeine  Biologie,  p.   29.     For  further  details  as  to  the  constituents 
of  the  nucleus,  see  pp.  29-44. 

2  Cf.  J.  Reinke,  Philosophic  der  Botanik,  1903,  pp.  69  and  72. 


It  may  be  asked  why  different  parts  of  the  cell  behave  in 
such  different  fashions,  when  the  same  stain  is  applied  to  them, 
and  so  render  it  possible  for  us  to  penetrate  into  the  mysteries 
of  its  structure.  Two  theories  have  been  put  forward  to 
account  for  this  behaviour.  According  to  one,  which  is  known 
as  the  chemical  theory  of  stains,  it  is  assumed  that  the  degree 
of  readiness  with  which  the  various  parts  of  the  cell  take  a 
stain  depends  upon  the  amount  of  chemical  affinity  existing 
between  the  various  albuminous  compounds  and  the  stain 
applied.  According  to  the  other  and  newer  theory,  certain 
parts  of  the  cell  are  susceptible  to  stain,  only  because  of  the 
changing  physical  qualities  of  the  thing  stained,  and,  as  a 
result,  its  powers  of  absorption  vary.  Alfred  Fischer  is  the 
chief  supporter  of  this  physical  theory.1  It  seems  probable 
that  both  theories  are  more  or  less  true,  and  that  the  staining 
capacity  of  the  various  morphological  elements  of  the  cell 
may  be  ascribed  partly  to  chemical  and  partly  to  physical 

In  close  connexion  with  his  examination  of  the  effects  of 
fixing  and  staining  upon  the  substance  of  a  living  cell,  A. 
Fischer  has  propounded  a  new  theory,  which  he  designates 
that  of  the  polymorphism  or  pleomorphism  of  protoplasm.3 
He  believes  protoplasm  to  be  in  general  viscous,  containing 
structures  of  various  shapes,  granular  or  reticular,  some  of 
which  remain  permanently,  whilst  others  are  of  a  transitory 
nature.  All  these  varieties  in  the  cell-framework  are  due  to 
definite  albuminous  compounds  fluctuating  between  a  fluid 
and  a  solid  condition.  Moreover,  Fischer  is  of  opinion  that 
protoplasm  is  often  homogeneous  on  the  surface,  but  in  the 
interior  occur  granules,  filaments,  reticular  framework,  and 
occasionally  also  Butschli's  alveolar  structures.  Fischer  is 
not  a  supporter  of  the  absolute  homogeneity  of  protoplasm, 
for  in  the  face  of  ascertained  facts  this  can  no  longer  be  defended, 
but  he  admits  that  the  various  cellular  structures  observed  by 
modern  scientists  are,  at  least  to  a  great  extent,  not  artificial 
products,  i.e.  the  results  of  staining  and  fixing,  but  occur 
also  in  the  living  cell.  He  does  not,  however,  believe  that 

1  Fixierung,  Farlung  und  Ban  des  Protoplasmas,  Jena,  1899. 

2  We  find  similar  ideas  in  Yves  Delage's  La  structure  du  protoplasma  et 
les  theories  sur  Vheredite,  pp.  30  and  31. 


these  structures  point  to  any  chemical  difference  in  the  parts 
of  the  cell,  but  are  the  outcome  of  the  physical  conditions 
affecting  the  protoplasm  at  any  given  moment.  Fischer 
obviously  does  not  intend  to  deny  the  complex  chemical  com- 
position of  living  substance,  but  he  doubts  whether  there  is 
any  necessary  connexion  between  the  chemical  constitution  of 
the  parts  of  the  cell  and  their  staining  capacity — such  a  con- 
nexion as  would  justify  our  assuming  that  a  chemical  difference 
exists  between  parts  that  show  a  different  staining  capacity. 

Although  Fischer's  theory  of  the  polymorphism  of  proto- 
plasm has  a  good  deal  that  is  hypothetical  about  it,  there  is 
far  more  actual  foundation  for  it  than  for  Altmann's  granular 
theory  ;  in  fact,  the  latter  bears  the  character  of  a  phylogenetic 
speculation  rather  than  that  of  a  scientific  theory.  The  theory 
of  the  polymorphism  of  protoplasm  has  one  great  advantage, 
viz.  that  it  reconciles  the  conflicting  opinions  regarding  the 
morphological  structure  of  the  cell  with  one  another,  and 
supplies  one  uniform  explanation  of  the  actual  variety  of 


What,  then,  is  the  morphology  of  the  cell  in  the  light  of 
modern  research  ?  This  question  can  be  answered  best,  if 
we  glance  back  at  the  views  regarding  the  structure  of  the 
cell  that  have  been~  current  at  various  stages  of  cytological 
research.  They  may  be  represented  by  the  diagram  on  p.  64 
(figs.  3-6).i 

Fig.  3  is  a  cell  as  Malpighi  (1678)  and  Wolff  (1759)  conceived 
it ;  it  consists  simply  of  the  enclosing  membrane,  and  so  is 
nothing  but  an  empty  sac. 

Fig.  4  is  a  cell  such  as  Schleiden  and  Schwann  described 
(1838-9).  The  membrane  is  still  an  essential  part,  but  it  is 
now  partly  filled  with  fluid,  in  which  is  suspended  another 
essential  part,  viz.  the  nucleus,  with  one  nucleolus. 

Fig.  5  is  the  cell  according  to  Ley  dig  (1857)  and  Max 
Schultze  (1861).  The  viscous  fluid  fills  the  whole  sac,  and 

1  Cf.  M.  Duval,  Precis  d' Histologie,  1900,  pp.  25,  31.  Also  G.  Schlater,  'Der 
gegenwartige  Stand  der  Zellenlehre  '  (Biolog.  Zentralblatt,  XIX,  1899,  p.  756). 



surrounds  the  nucleus  and  its  nucleolus,  but  the  membrane 
has  disappeared  as  not  essential  to  the  existence  of  the  cell. 
Subsequently  the  finer  structure  of  the  cell  was  more  closely 
examined,  and  the  mass  of  apparently  homogeneous  proto- 
plasm was  seen  to  be  a  compound  formation,  consisting  of 
framework  and  fluid,  whilst  the  nucleus,  too,  was  found  to 
contain,  besides  the  nucleolus,  an  achromatic  framework 
embedded  in  nuclear  fluid,  and  also  a  chromatin  framework 
that  assumes  various  forms.  We  may  connect  the  names  of 

FIG.  3. 

FIG.  4. 

FIG.  5. 

FIG.  6. 

Schlater,  Keinke,  and  Waldeyer  with  this  stage  of  cellular 
morphology  (1894-5). 

Fig.  6  represents  it  according  to  Carnoy,1  who  regards  the 
cellular  framework  as  reticular,  and  the  chromatin  nuclear 
framework  as  consisting  of  a  coil  of  nuclein-plastin  thread.3 
This  conception  of  the  cell  harmonises  best  with  my  own 
cytological  examination  of  the  huge  pericardial  3  cells  of  the 
Termitoxenia  (Termitomyia)  mirabilis. 

1  Carney's  valuable  work  in  the  development  of  cytology  has  been  already 
mentioned.     See  p.  46. 

2  Cf.  also  E.  B.  Wilson,  The  Cell,  p.  35.     Fig.  13A  is  an  admirable  representa- 
tion of  a  permanent  spireme  nucleus,  showing  chromatin  in  a  single  thread 

3  This  is  the  name  given  to  some  peculiar  cells,  allied  to  the  adipose  cells, 
and  connected  with  the  '  heart '  of  the  insect,  i.e.  with  its  vas  dorsale. 


Within  the  chromatin  thread  of  the  nuclear  framework  it 
is  possible  in  many  cases  to  perceive  a  still  finer  morphological 
differentiation.  In  the  American  salamander  Batrachoseps 
the  threads  are  plainly  divided  and  each  pronucleus  contains, 
according  to  Gustav  Eisen,  twelve  chief  parts  or  chromosomes. l 
Each  chromosome  as  a  rule  is  subdivided  into  six  chromomeres, 
in  each  of  which  on  an  average  six  of  the  most  diminutive 
bodies  or  chromioles  can  be  traced.  There  are  therefore  about 
400  distinguishable  parts  in  the  chromatin  thread  of  the  nucleus  ! 

There  are  also  other  animal  and  vegetable  cells,  which,  before 
division,  show  only  a  coil  of  chromatin  thread,  or  a  chromatin 
framework,  but,  in  the  course  of  indirect  or  mitotic  division,  this 
develops  into  definite  groups  of  chromatin  knots  or  chromo- 
somes ;  whilst  within  the  achromatic  framework,  that  was 
previously  scarcely  visible,  there  now  appear  as  organs  of  cell- 
division  tiny  round  centrosomes,  in  the  midst  of  which  rises  an 
achromatic  spindle.  All  these  phenomena  will  be  discussed 
more  fully  in  Chapters  V  and  VI,  for  they  do  not  properly 
belong  to  the  morphology  of  the  resting  cell,  or  cell  not  in 
process  of  division. 

The  cell  is  therefore  far  from  being  a  simple  formation  ; 
it  is,  on  the  contrary,  composed  of  parts  differing  widely  from 
one  another,  and  having  different  functions  in  its  life.  We 
have  now  to  consider  the  chief  kinds  of  activity  in  the  cell, 
and  the  parts  taken  in  this  activity  by  the  morphologically 
different  elements  of  it,  and  then  we  shall  be  in  a  position  to 
discuss  the  question  whether  the  cell  is  the  ultimate  unit  in 
organic  life,  or  whether  it  is  equivalent  to  an  aggregate  of  still 
more  simple  and  elementary  units.  A  result  of  this  discussion 
will  be  to  show  us  what  ought  to  be  our  attitude,  as  students  of 
natural  science,  towards  the  famous  theory  of  the  spontaneous 
generation  of  organic  beings. 

1  Pronucleus  is  the  name  given  to  the  nucleus  of  both  the  egg-  and  sperm- 
cells  immediately  after  their  union  in  the  process  of  fertilisation.  See 
Chapter  VI. 




Division  of  labour  among  cells  (/;.  68).  Life  a  process  of  movement 
directed  to  a  material  end  (p.  69). 


Phenomena  of  movement  in  Amoebae  and  other  Rhizopods  (p.  70). 
Life  and  work  of  the  white  blood-corpuscles  (leucocytes)  (p.  72). 


Cilia  and  flagella  as  external  organs  of  movement  belonging  to  the  cell 
(p.  74).  Interior  products  of  the  cell.  Various  biochemical 
departments  of  work.  Biological  importance  of  fat  and  of 
haemoglobin  (p.  75). 



Vivisection  of  unicellular  animals  and  plants  (p.  80).  The  nucleus  the 
central  point  of  the  vital  processes  in  the  cell  (p.  83). 



CELLS  are  the  bricks  composing  the  whole  building  of  the 
organic  world.  Therefore  to  them  also  is  the  Creator's  com- 
mand addressed  :  '  Increase  and  multiply/  for  without  growth 
and  multiplication  of  cells  no  organic  life  is  conceivable.  All 
living  creatures  consist  of  one  or  more  cells  ;  if  they  are  uni- 
cellular, increase  is  possible  only  if  from  one  cell  several  cells 
are  formed  ;  if  they  are  multicellular,  growth  and  increase 
are  possible  only  by  way  of  growth  and  increase  of  the  cells 
composing  their  organs  and  tissues. 

In  the  previous  chapter  we  discussed  the  structure  of  the 
resting  cell,  as  revealed  to  us  by  modern  microscopical  research  ; 
we  have  now  to  turn  our  attention  to  the  cell  as  active  and 
alive.  In  the  case  of  unicellular  animals  and  plants,  the 
diminutive  mass  of  protoplasm  with  its  one  nucleus  is  the  one 
organ  that  has  to  discharge  all  the  functions  of  life  ;  it  is, 
to  compare  small  with  great,  a  Jack-of-all  trades  in  the  economy 
of  life.  Nutrition  and  multiplication,  as  well  as  independent 
movement  and  sensation  (as  far  as  these  latter  manifest  them- 



selves  in  unicellular  creatures),  all  depend  upon  one  and  the 
same  atom  of  living  substance.  It  is  true  that  here,  in  spite 
of  the  diminutive  size  of  the  creature  under  consideration, 
we  have  something  analogous  to  what  is  called  '  organisation  '  in 
higher  animals,  for,  as  we  shall  show  later  on,  the  morphologi- 
cally different  parts  of  the  cell  have  various  functions.  Still, 
strictly  speaking,  the  parts  of  the  cell  ought  not  to  be  called 
organs,  although,  perhaps,  we  may  follow  some  recent  writers 
and  call  them  organellae,  at  least  when  speaking  of  the  multi- 
cellular  animals  known  as  metazoa.  In  their  case,  whenever 
we  use  the  word  organ,  we  mean  some  part  consisting  of  definite 
tissues  and  serving  as  an  instrument  in  the  vital  activity  of  an 
individual.  As  the  tissues  are  made  up  of  cells,  which  are 
therefore  the  ultimate  constituents  of  the  organs,  it  would  be 
logically  wrong  to  apply  the  same  word  '  organs  '  to  the  smallest 
parts  of  the  cells  themselves.  It  has  lately  become  too  much 
the  custom  to  disregard  the  connecting  membrane  which  unites 
cells  together  to  form  tissues,  and  tissues  to  form  organs.  The 
result  of  this  has  been  that,  in  both  the  higher  animals  and  plants, 
the  cell  has  come  to  be  regarded  as  having  an  independent 
existence,  as  being  an  individual  of  a  lower  order.  This  view  is, 
however,  altogether  mistaken,  and  it  is  no  less  wrong  to  apply 
the  name  '  organs  '  to  the  minute  constituents  of  the  cell, 
which  differ  morphologically  and  physiologically.  If  they  are 
organs  at  all,  they  are  so  only  in  a  loose,  metaphorical  sense. 

It  is  only  in  the  case  of  unicellular  organisms  that  this 
theoretical  opinion  corresponds  with  facts,  for  in  them  the 
constituent  parts  of  the  cell  really  discharge  the  vital  functions 
of  the  individual,  and  so  are  equivalent  to  the  organs  of  multi- 
cellular  organisms.  For  this  reason  the  unicellular  organisms 
form  the  lowest  rung  of  the  ladder  of  organic  perfection.  The 
higher  we  ascend,  the  more  are  the  various  parts  differentiated 
to  perform  distinct  functions,  and  the  greater  is  the  perfection 
of  the  organisation.  A  vertebrate  animal,  or  even  a  tiny 
insect,  is  a  well-ordered  and  regulated  state,  whose  inhabitants 
and  officials  are  thousands  and  tens  of  thousands  of  cells.1 

1  The  reader  must  notice  that  this  expression  is  figurative.  In  reality, 
as  has  been  already  pointed  out,  the  cells  of  a  multicellular  organism  are  not 
individuals,  because  they  are  not  physiological  units  complete  in  themselves, 
as  are  unicellular  organisms.  On  this  subject  see  Chapter  VII,  §  1  :  '  The  cell 
as  the  ultimate  unit  in  organic  life.'  Cf.  also  0.  Hertwig,  Allgemeine  Biologic, 
1906,  chapters  14-17. 

F  2 


All  are  democrats,  for  none  is  of  higher  origin  than  the 
others ;  the  nerve-cell  of  the  brain,  which  exercises  control, 
like  the  ruler  of  the  state,  is  a  cell  in  exactly  the  same  way 
as  the  glandular  cell  of  the  stomach,  or  the  epithelial  cell 
of  the  skin.  But  in  spite  of  their  genuinely  democratic  disposi- 
tion, the  cells  are  by  no  means  anarchists  ;  there  prevails 
among  them  a  most  perfect  harmony,  based  upon  a  regular 
division  of  labour  between  the  various  organs,  tissues,  and 

Just  as  in  every  well-ordered  state  different  duties  are 
assigned  to  different  officials,  so  to  various  organs  are  assigned 
the  functions  of  nutrition,  digestion,  circulation  of  the  blood, 
respiration,  propagation,  movement  and  all  the  work  done  by 
the  nerves  and  senses.  But  these  organs,  which  resemble  the 
heads  of  departments  in  the  state,  are  themselves  made  up 
of  different  kinds  of  subordinate  tissues,  and  each  tissue  con- 
sists of  a  more  or  less  varied  combination  of  cells,  differing 
in  the  case  of  the  different  tissues.  All  these  millions  of  cells 
compose  what  we  call  an  organism,  and  in  spite  of  their  vast 
number  and  endless  variety  they  all  have  the  same  origin, 
for  they  all  proceed  from  an  egg-cell  fertilised  by  means  of  a 
spermatozoon  ;  such  at  least  is  the  ordinary  process  of  develop- 
ment of  any  higher  organism.2 

The  continuation  of  the  process  of  cleavage,  begun  in  the 
first  cleavage  or  segmentation  nucleus,  leads  eventually  to  a 
differentiation  of  the  living  creature  into  various  cells,  tissues 
and  organs,  until  it  attains  its  full  development,  and  then 
the  work  of  propagation  renews  the  cycle  of  life.  But  even 
the  egg-cells  and  the  spermatozoa,  although  they  carry  on  the 
task  of  propagation,  differ  in  no  respect  from  other  cells,  as  far 
as  their  origin  is  concerned  ;  in  the  course  of  embryonic 
development  they  are  differentiated  from  common  cells, 
into  which  the  fertilised  egg  split  up  at  the  formation  of  the 
periphery  of  the  embryo.3 

1  On  the  subject  of  the  division  of  labour  in  an  aggregation  of  cells,  see 
0.  Hertwig,  chapter  17,  pp.  417,  &c. 

2  I   sayy  ordinary,'  because  of  the  phenomena  of  parthenogenesis  among 
insects,   &c.,  where  the  egg-cell  develops  without  fertilisation.    (See  Chapter 
VI,  §6.) 

3  See  Chapter  VI,  §  3,  for  the  most  recent  results  of  investigations  regarding 
the  distinction  between  somatic  and  germ  cells,  which  is  either  very  early 
or  even  original. 


All  the  cells,  therefore,  in  the  organism  enjoy  absolute 
'  equality  before  the  law,'  but  it  is  an  equality,  not  of  death 
but  of  active  life,  inasmuch  as  from  cells,  at  first  similar,  the 
mysterious  laws  of  organic  development  produce  the  living 
being  in  all  its  wonderful,  complete,  and  complex  structure. 

Such  is  in  outline  the  cellular  life  of  the  multicellular 
organism,  which  we  cannot  now  discuss  in  greater  detail. 
What  has  been  said  will  suffice  to  show  that  the  cell  must  be 
called  the  lowest  unit  of  organic  life  in  multicellular  animals 
and  plants.  Let  us  now  study  more  closely  the  vital  processes 
affecting  cells  as  such,  whether  they  are  united  to  form  tissues 
of  a  higher  order,  or  lead  an  independent  existence  as  unicellular 
beings.  This  study  will  give  us  a  deeper  insight  into  the  real 
nature  of  the  cell,  this  marvel  of  creation. 

Life  is,  in  its  physiological  aspect,1  an  uninterrupted 
process  of  movement,  every  phase  of  which  tends  to  the  pre- 
servation of  the  individual  and  of  the  species.  The  interior 
movements,  which  form  the  really  essential  processes  of 
vegetative  life,  tend  to  the  assimilation  of  fresh  material,  and 
so  to  the  growth  of  the  individual.  These  processes  of  assimila- 
tion, depending  as  they  do  upon  nutrition  and  respiration,  are 
necessarily  closely  connected  with  analogous  phenomena  of 
dissimilation,3  for  the  building  up  of  what  is  new  requires  a 
tearing  down  of  what  is  old,  and  the  reception  of  fresh  nutritive 
matter  and  its  transformation  into  living  substance  necessitate 
a  removal  of  what  is  worn  out.  Growth  is  based  upon  assimila- 
tion and  leads  naturally  to  numerical  increase.  As  soon  as  a 
cell  has  reached  a  definite  maximum  size,  it  divides  and  forms 
new  cells  ;  if  these  remain  united  in  one  aggregate  of  tissues, 
the  division  of  the  cell  promotes  the  growth  of  the  individual ; 
if,  however,  the  new  cells  separate  from  the  parent  organism, 
so  as  to  form  new  independent  individuals,  then  the  division 
of  the  cell  is  a  process  of  propagation,  and  furthers  the 
preservation  of  the  species.  To  these  interior  processes  of 
movement  in  the  living  substance  correspond  other  exterior 

1  For  further  details  regarding  the  physiology  of  the  vital  processes,  the 
nutrition  and  transmutation  of  energy  of  cells,  and  the  processes  of  assimi- 
lation and  dissimilation,  see  Bunge,  Physiologische  Chemie,  and  J.  Reinke, 
Einleitung  in  die  theoretische  Biologic,  chapters  26-29. 

-  The  word  dissim  lation  was  introduced  by  Hering  as  an  euphonious  abbre- 
viation of  des-assimilation,  which,  being  a  clumsy  word,  is  now  but  little  used. 


movements,  due  to  the  susceptibility  of  protoplasm  to  definite 
external  stimuli ;  these  latter  movements  tend  to  procure  the 
material  necessary  to  support  the  interior  vital  processes, 
whether  it  be  by  the  assimilation  of  food  to  promote  individual 
growth,  or  by  the  union  of  individuals  to  promote  the  preserva- 
tion of  the  species  ;  finally,  the  exterior  movements  protect 
the  organism  from  its  enemies.  Thus  all  the  exterior  move- 
ments are  subservient  to  the  interior,  even  when,  as  voluntary, 
they  belong  to  conscious  existence,  and  therefore  are  on  a  higher 
level  than  the  vegetative  processes,  for  the  whole  conscious 
life  of  an  animal  aims  at  the  preservation  of  the  individual 
and  of  the  species  ;  it  stands  to  living  matter  in  the  position 
of  a  slave  ;  its  sole  aim  is  material,  and  it  has  no  power  to  rise 
above  the  material,  as  the  intellectual  life  of  man  enables  him 
to  do. 


The  foregoing  general  observations  will  enable  us  to  under- 
stand the  phenomena  that  we  are  now  about  to  consider. 

Oskar  Hertwig  in  his  '  Allgemeine  Biologie,'  pp.  108,  &c., 
recognises  several  distinct  kinds  of  movement  in  protoplasm, 
and  we  may  safely  follow  him  on  this  point,  Keal  protoplasmic 
movement  either  belongs  to  a  complete  protoplasmic  body, 
such  as  an  amoeba,  or  it  takes  place  in  the  interior  of  a  cellular 
membrane.  This  latter  form  of  movement  occurs  chiefly  in 
plants,  and  is  divided  into  rotatory  and  circulatory  move- 
ments. The  rotatory  movement  was  discovered  by  Bona- 
ventura  Corti  as  early  as  1774.  We  must  distinguish  these 
genuine  movements  of  protoplasm  from  those  due  to  exterior 
appendages  on  the  cells,  such  as  cilia  and  flagella,  with  which 
we  shall  deal  in  the  next  section  of  this  chapter.  We  must 
refer  also  to  the  movements  of  pulsating  vacuoles  in  unicellular 
animals,  and  to  the  manifold  passive  alterations  in  shape  and 
position  undergone  by  the  cells  of  an  organism  in  consequence 
of  the  vital  process  going  on  within  it  as  a  whole.  At  present, 
however,  we  are  concerned  only  with  a  few  instances  of  true 
protoplasmic  movement. 

The  protoplasm  of  a  living  cell  is  in  a  state  of  constant 
activity,  and  moves  on  definite  lines  inside  the  cell,  its  course 


being  apparently  determined  by  the  framework  of  spongio- 
plasm.  At  the  end  of  the  eighteenth  and  at  the  beginning  of 
the  nineteenth  century  Corti  and  Treviranus  noticed  (see  p.  33) 
that  the  chlorophyll  granules,  which  give  plants  their  green 
colour,  are  frequently  in  vigorous  movement  within  the  cells  ; 
later  on,  in  1848,  von  Mohl  discovered  this  granular  movement 
not  to  be  active,  but  passive,  and  due  to  the  power  of  contrac- 
tion possessed  by  protoplasm.  In  many  of  the  lower  animals 
protoplasm  appears  capable  of  active  movement,  but  we  must 
be  careful  to  distinguish  two  forms  of  activity — the  active 
movement  of  the  protoplasm  framework,  that  manifests  itself 
especially  in  external  changes  of  shape,  and  a  more  passive 
flow  of  the  granules  in  the  cell-sap,  which  is  a  result  of  the 
contraction  and  expansion  of  the  protoplasmic  framework.  It 
is  obvious  that  these  processes  of  movement  cannot  always 
and  everywhere  be  traced  with  the  same  clearness  in  living 
cells.  They  can  be  seen  very  well  in  various  little  unicellular 
creatures  possessing  no  enclosing  membrane,  such  as  the 
Amoeba  proteus,1  and  still  better  in  other  animals  belonging  to 
the  same  class  of  Khizopods,  but  having  a  thin  shell,  through 
the  openings  of  which  the  so-called  pseudopodia  protrude,  as, 
for  instance,  in  the  case  of  the  Gromia  oviformis.* 

The  body  of  the  Amoeba  is  subject  to  constant  changes 
of  shape,  whence  the  creature  has  received  its  name.  It  can 
protrude  protoplasmic  continuations  of  its  substance  in  all 
directions  and  again  withdraw  them.  The  pseudopodia  are 
outstretched  to  catch  food  and  to  effect  a  change  of  place  ; 
they  are  withdrawn  when  any  danger  threatens.  If  the 
pseudopodia  of  an  Amoeba  are  fed  with  very  small  grains  of 
carmine,  these  grains  are  at  once  surrounded  by  the  proto- 
plasm of  the  pseudopodia  and  absorbed  by  it,  and  then  they 
share  in  the  interior  flow  of  the  protoplasm  and  render  it 
visible  under  the  microscope.  In  Amoebae  there  is  no 
sharp  distinction  between  interior  and  exterior  movements, 
for  both  are  nothing  but  the  same  flow  of  the  same  protoplasm. 
When  the  pseudopodia  discover  anything  edible  they  close 
round  it,  and  it  at  once  becomes  the  centre  of  a  vortex  of 

1  The  changes  of  shape  undergone  by  this  little  Amoeba  were  described 
as  early  as  1755  by  Roesel  von  Rosenhof. 

2  Within  the  pseudopodia  of  true  Amoebae  no  movements  can   be  dis- 
cerned, although  they  occur  in  the  other  Rhizopods. 


protoplasm,  for  the  creature's  whole  body  contracts  round  its 
prey.  The  same  protoplasm,  which  sought  and  captured  its 
food,  now  proceeds  to  assimilate  it,  and  digests  as  much  of  it 
as  is  digestible,  and  then  rejects  the  rest  by  uncoiling  the 
enclosing  ring  of  protoplasm. 

More  vigorous  movements  than  those  of  the  Amoeba  can 
be  observed,  as  already  stated,  in  the  pseudopodia  of  many 
other  Khizopods,  especially  the  Foraminifera  and  Eadiolaria, 
which  possess  a  solid  skeleton  of  chalk  or  silica,  and  through 
its  openings  protrude  the  long  pseudopodia  in  quest  of  food  or 
to  effect  change  of  place. 

Amoeboid  movements  as  well  as  the  granular  flow  of  proto- 
plasm may  be  produced,  checked,  and  altered  by  mechanical, 
chemical  and  thermal  stimuli,  and  this  constitutes  the  chief 
proof  of  the  irritability  of  living  protoplasm. 

Analogous  to  the  action  of  the  Amoebae  and  their  relations 
in  the  wrater  is  that  of  some  cells  in  the  organism  of  multi- 
cellular  animals,  especially  of  the  white  blood-corpuscles  or 
leucocytes.  They  too  possess  amoeboid  prolongations,  enabling 
them  to  move  and  traverse  all  the  tissues  of  the  body.  In  order 
to  pass  through  a  narrow  crevice,  they  put  out  a  pseudopodium 
first,  and  gradually  the  whole  body  of  the  cell  follows  it. 
Cohnheim,  who  discovered  the  power  of  the  leucocytes  to 
wander  through  the  tissues  of  the  body,  bestowed  upon  it  the 
very  suitable  name  of  Diapedesis.  These  wandering  cells  have 
an  almost  insatiable  appetite  ;  they  are  like  tramps,  always 
hungry  and  thirsty,  and  they  attack  other  cells,  as  well  as 
any  extraneous  substances  that  have  penetrated  into  the  body, 
and  encounter  them  on  their  way.  The  leucocytes  surround 
these  on  all  sides  and  devour  them,  hence  their  other  name 
of  Phagocytes.  Their  voracity  gives  them  a  high  degree  of 
importance  in  the  life  of  the  organism.  The  white  blood- 
corpuscles  discover  the  red  blood-corpuscles  that  are  old  and 
incapable  of  taking  up  oxygen,  and  seize  them  and  carry  them 
off,  and  thus,  by  consuming  the  useless  members  of  the  com- 
munity of  cells,  the  leucocytes  are  able  to  impart  the  nourish- 
ment so  obtained  to  other  active  formative  elements  of  the 
body.  They  are  the  police,  appointed  to  keep  order  in  the 
cell-republic  that  we  call  an  organism.  They  go  to  and  fro 
through  all  the  tissues  and  purify  them  from  hostile  bacilli 


and  other  wrongdoers.     Whenever  they  light  upon  anything 
harmful,  they  simply  close  round  it  and  devour  it ;   or,  if  it  is 
altogether  inedible,  e.g.  a  speck  of  coal  dust,  they  arrest  it  and 
drive  it  over  the  frontier.     The  leucocytes  are  therefore  real 
sanitary  inspectors  in  the  organisms  of  man  and  the  higher 
animals.     Many  authors  ascribe  to  their  agency  the  assimilation 
of  the  nutritive  matter  absorbed  in  the  intestinal  glands,  as 
well   as   the   diffusion  of  nourishing  lymph   throughout   the 
whole  body,1  and   from   this    point   of   view   the   wandering 
leucocytes  appear  as  nurses,  supplying  food  to  the  other  cells 
and    tissues.     On   the   other   hand,   however,  under   certain 
morbid  conditions,  leucocytes  increase  with  such   overpower- 
ing rapidity   as   to   become   dangerous.     They    then   attack 
cells  that  ought  to  be  left  in  peace,  and  so  excite  a  kind  of 
revolution  resulting  in  inflammation  and  suppuration  of  the 
tissues,  and  tending  to  the  eventual  destruction  of  the  whole 
organism.     In  spite,  therefore,  of  their  physiological  merits, 
leucocytes    have    acquired    a    bad    reputation    in    cellular 
pathology.     Moreover,  the  most  recent  investigations  carried 
on    by    Ehrlich,    Metchnikoff    and    others    have    deprived 
leucocytes  of  many  of  the  police  functions  generally  ascribed 
to  them.     According  to  the  most  modern  views,  the  struggle 
between  health  and  disease  is  fought  out  chiefly  by  toxins  and 
antitoxins,  the  former  being  chemical  substances  injurious  to 
the  organism,  and  given  off  by  harmful  bacteria,  &c.,  whilst 
the  latter  are  the  chemical  antidotes,  produced  by  the  organism 
itself  as  a  protection  against  toxins.     Modern  processes  of 
inoculation  aim  at  causing  immunity  from  certain  diseases  by 
producing  specific  antitoxins. 

A  harmless  counterpart  to  the  pathological  action  of 
leucocytes  in  the  bodies  of  men  and  the  higher  animals  occurs 
in  the  phagocytes  of  those  insects  which  undergo  a  complete 
metamorphosis.  To  these  cells  is  assigned  the  pleasing  task 
of  devouring  the  old  tissues  of  the  larval  body  during  the  pupal 
stage,  in  order  to  impart  the  stored-up  nutritive  matter  to 
other  cells  concerned  in  the  formation  of  the  new  tissues  of  the 

A  flow  of  protoplasm  occurs  also  in  cells  where  it  has 
deposited  an  exterior  membrane  and  cannot  therefore  protrude 

1  Cf.  M.  Duval,  Precis  d'Histologie  (1900),  p.  42. 


pseudopodia,  but  in  this  case  the  movements  are  limited  to 
the  interior  of  the  cell.  This  movement  of  protoplasm  in 
plant  cells  has  long  been  known  to  botanists  and  often  described, 
for  instance,  in  the  leaf  cells  of  the  Elodea  canadensis  and  in 
the  stamens  of  the  Tradescantia,  &c. 


Just  as  the  activity  of  the  protoplasm  inside  a  cell  enables 
it  to  form  a  solid  membrane  as  its  envelope,  so  it  can  produce 
movable  processes  on  the  surface  of  the  cell,  such  as  cilia  and 
flagella,  which  facilitate  the  locomotion  of  the  cell.  In  this  way 
ciliated  and  flagelliform  cells  arise.  The  latter  have  either 
one  or  a  few  long,  thick  processes,  whilst  the  former  have  rows 
of  delicate  hair-like  threads.  Among  the  Infusoria  there  is  a 
class  of  unicellular  creatures  called  Flagellata,  from  their 
having  these  flagelliform  processes,  and  another  class  of 
Protozoa  is  known  as  Ciliata,  because  their  cell-walls  are 
provided  with  cilia,  which  enable  them  to  move  about  in  the 
water.  Cilia  are  important  in  the  ingestion  of  food,  for 
these  creatures,  though  unicellular  and  of  diminutive  size, 
have  voracious  appetites.  The  ring  of  cilia  surrounding  the 
oral  aperture  of  an  infusorian  by  its  rhythmical  motion  produces 
a  vortex  in  the  water,  at  the  centre  of  which  is  the  mouth  of 
the  little  animal.  If  a  tiny  diatom  or  another  of  the  Algae 
is  caught  in  this  vortex,  it  has  no  chance  of  escape ;  it  is  sucked 
down  and  vanishes  in  this  Scylla,  and  only  its  indigestible 
remains  are  eventually  thrown  up. 

Flagelliform  and  ciliated  cells  occur  also  in  multicellular 
animals.  Spermatozoa  are  simple  flagelliform  cells,  of  which 
the  nucleus  forms  the  head,  and  a  long  thread  of  protoplasm 
the  body  and  tail.  Ciliated  cells  occur  chiefly  in  the  respiratory 
and  digestive  apparatus,  and  in  this  case  the  cilia  do  not  assist 
in  the  movement  of  the  cell  to  which  they  are  attached,  but 
in  that  of  the  substance  passing  over  them.  The  cilia  of  the 
trachea  serve  to  expel  small  foreign  bodies  that  have  entered 
the  respiratory  orifices,  and  those  of  the  oesophagus  help  to 
carry  down  the  nutritive  fluids  taken  in  through  the  mouth, 
and  to  keep  them  in  steady  movement  towards  the  digestive 

1  See  0.  Hertwig,  Allgemeine  Biologie,  1906,  pp.  79,  &c.,  pp.  100,  &c. 


organs.  In  many  of  the  higher  and  lower  animals  ciliated 
cells  occur  in  the  real  digestive  canal.  I  have  seen  very 
beautiful  ones,  magnified  1500  times,  in  the  transverse  sections 
of  the  mesenteron  of  the  Termitoxenia  (Termitomyia)  Braunsi. 

The  outward  or  exoplasmic  products  of  the  cell  are  the 
external  results  of  the  internal  activity  of  the  protoplasm. 
They  may  take  the  form  of  a  cellular  membrane,  whether  it  is 
homogeneous  with  the  protoplasm  (as  is  the  case  with  most 
animal  cellular  membranes),  or  whether  it  is  a  chemical  product 
of  protoplasm,  as  is  the  case  with  the  cellulose  cell-walls  of 
plants,1  or  the  shells  of  many  of  the  lower  animals  (e.g.  the 
Foraminifera)  or  the  coverings  of  plants  (e.g.  the  Diatomaceae) 
which  have  been  hardened  by  taking  up  silicic  acid  or  carbonate 
of  lime.  Further  exoplasmic  products  of  the  cell  are  the 
elastic  intercellular  bridges  uniting  cells  with  one  another, 
arid  the  cilia  and  flagella  which  pro  trade  from  the  cellular 

The  internal  or  endoplasmic  products  of  the  cell  are 
contained  in  its  interior.  They  are  of  most  frequent  occurrence 
in  the  vegetable  kingdom.  In  the  chemical  laboratory  of  the 
living  plant  cell  grains  of  starch  are  being  prepared  which 
supply  the  world  with  sugar,  either  directly,  or  indirectly 
through  the  activity  of  the  plant.  Starch  is  the  form  in  which 
the  plant  stores  up~  the  carbo-hydrates  that  produce  sugar. 
The  protoplasm  of  plants  was  believed  to  form  chlorophyll 
under  the  influence  of  light,  thus  giving  its  colour  to  the  foliage  ;  3 
but  recently  many  scientists  have  inclined  to  the  opinion  that 
chlorophyll  is  not  a  cellular  product,  and  that  its  presence,  not 
only  in  many  lower  animals,  such  as  the  Hydra  viridis,  but 
also  in  plants,  is  due  to  a  symbiosis  of  special  chlorophyll 
cells  with  other  vegetable  or  animal  cells.3 

1  The  young  membrane  of  a  plant  cell  consists  alwa}^  of  cellulose,  but 
in  many  instances  the  cell-  walls  harden  later  on  into  cork  or  wood. 

2  The  granules  which  convey  the  colouring  matter  originate  in  the  plant  cell 
even  without  the  influence  of  light,  although  the  green  colour,  which  can 
be  extracted  from  them,  only  develops  as  a  rule  when  light  is  admitted.     Young 
fir  trees  are  green,  however,  and  full  of  chlorophyll,  even  when  grown  in  the 
dark,  and  several  cryptogams  become  green  in  spite  of  complete  exclusion  of  light. 

3  Cf  .  G.  Mereschkowsky,  '  Uber  Natur  und  Ursprung  der  Chromatophoren 
im  Pflanzenreiche  '   (Biolog.  Zentralblatt,  XXV  (1905),  No.  18,  pp.  593-604). 
He  believes  the  Cyanophyceae  to  be  independent  chromatophores,  and  tries 
to  account  for  the  origin  of  the  vegetable  kingdom,  and  its  difference  from 
the  animal  kingdom,  by  assuming  that  they  have  penetrated  into  animal 
cells.     In  fact  a  lion,  sleeping  under  a  palm  tree,  would  change  places  with  it, 


Animal  and  vegetable  fat  is  a  product  of  the  interior 
activity  of  the  cell,  and  is  stored  up  in  its  empty  spaces.  In 
the  animal  kingdom  this  biochemical  branch  of  industry  is  of 
great  importance,  and  a  special  class  of  fat-forming  cells, 
called  adipose  cells,  often  make  up  large  quantities  of  tissue. 
In  their  vacuoles  little  drops  of  fat  collect  and  grow,  until 
finally  the  whole  cell  resembles  a  ball  of  fat  surrounded  by  a 
membrane.  The  neighbouring  cells  that  are  not  of  this  class 
can  feed  upon  this  stored-up  fat  by  way  of  endosmosis.  The 
protoplasmic  product  that  we  call  fat  is  of  great  importance  in 
the  nutrition  of  the  animal  organism.  It  used  to  be  regarded 
as  the  material  for  supplying  heat  in  the  process  of  combustion 
connected  with  respiration.  In  insects  fat  is  closely  connected 
with  the  formation  of  blood,  for  which  reason,  in  speaking  of 
them,  we  often  call  the  adipose  tissue  simply  the  blood-forming 
tissue.  I  found  many  instances  of  this  connexion  between 
fat  and  blood  in  the  course  of  my  microscopical  study  of  the 
inquilines  among  ants  and  termites,  and  especially  in  the 
physogastric  guests  of  the  termites,  which  rejoice  in  an  extra- 
ordinary abundance  of  fat.  In  the  larvae  of  the  termitophile 
beetle  of  Ceylon,  known  as  Orthogonius  Schaumi,  the  outer 
edge  of  the  huge  adipose  tissue  may  be  seen  just  at  the  spot 
where  it  touches  the  hypodermal  masses  of  blood,  and  it  is 
frequently  in  a  state  of  disintegration,  and  being  absorbed 
almost  imperceptibly  by  the  diminutive  corpuscles  of  the 
insect's  blood.  I  observed  similar  phenomena  in  other  genuine 
inquilines  among  the  termites,  which  become  physogastric 
through  their  abundance  of  adipose  tissue  ;  the  same  transition 
from  adipose  to  blood  tissue  appeared  on  a  series  of  sections 
of  a  termitophile  insect,  Xenogaster  inflata  of  Brazil.  The 
ants  and  termites  seem  to  appreciate  the  advantages  of  their 
guests'  adipose  tissue,  and  hold  to  the  dictum  Omne  pingue 
bonum  ;  for  all  their  true  inquilines,  belonging  to  the  class 
of  beetles,  possess  a  great  deal  of  fat,  and  it  is  this  tissue 
which  directly  or  indirectly  emits  the  volatile  exudation  that 
attracts  them  so  greatly  and  induces  them  to  lick  their  guests.1 

provided  the  cells  in  his  body  were  filled  with  chromatophores  (p.  604).     This 
is  certainly  a  very  hold  theory. 

1  Cf.  on  this  subject  '  Zur  naheren  Kenntnis  des  echten  Gastverhaltnisses 
bei  den  Ameisengasten  und  Termitengasten  '  (Biolog.  Zentralblatt,  XXIIT, 
1903,  Nos.  2,  5,  6,  7  and  8,  p.  68). 


There  are  a  number  of  other  products  of  the  interior  of 
the  cell  which  might  be  mentioned  ;  some  of  them  occur  in 
animal  cells  and  some  in  vegetable,  and  take  the  form  of 
essential  oils,  colouring  matters,  nectar,  caoutchouc  and 
india-rubber,  resin,  tannic  acid,  poisons  of  various  kinds, 
digestive  ferments,  &c.,  thus  serving  the  most  manifold  and 
interesting  biological  purposes. 

In  vertebrate  animals  the  haemoglobin  of  the  red  blood- 
corpuscles  is  one  of  the  products  of  the  interior  of  the  cell. 
This  haemoglobin,  to  which  blood  owes  its  colour,  carries  the 
life-giving  oxygen  which  we  breathe  in  ;  the  molecules  of 
oxygen  are  brought  through  the  lungs  into  the  blood,  and 
accompany  the  red  blood-corpuscles  over  the  whole  extent  of 
the  arterial  circulation,  making  their  way  through  the  finest 
capillary  vessels  to  the  single  cells  of  the  tissues,  where  they 
give  out  their  oxygen  and  so  oxydise  the  existing  organic 
connexions.  The  free  carbonic  acid,  which  is  the  chief 
combustion  product  of  the  vital  process,  has  now  to  be  expelled 
from  the  body  by  the  same  means  ;  so  the  red  blood-corpuscles 
are  accompanied  by  carbonic  acid  molecules  on  their  way 
back  from  the  capillary  vessels,  through  the  whole  extent  of 
the  venous  circulation,  until  they  reach  the  lungs,  where 
the  carbonic  acid  is  breathed  out  into  the  air,  and  at  the  next 
inspiration  fresh  oxygen  is  taken  up,  to  join  the  red  blood- 
corpuscles  on  their  next  journey  through  the  body.  The 
arterial  and  the  venous  blood  differ  in  colour  because  the 
haemoglobin  of  the  red  blood-corpuscles  forms  a  soluble 
chemical  combination  with  the  oxygen,  producing  bright 
red  oxyhaemoglobin,  whilst  the  same  blood-corpuscles,  after 
giving  off  their  oxygen  to  the  cells  of  the  body,  resume  their 
previous  dark  bluish-red  tint. 


We  have  now  considered  some  characteristic  instances  of 
the  processes  of  cell-nutrition,  cell-growth,  and  cell-motion. 
Before  passing  on  to  a  new  and  important  class  of  phenomena 
of  cellular  life,  viz.  the  process  of  multiplication  by  cell-division, 
we  must  examine  more  closely  the  part  played  by  the  nucleus 


in  the  manifestations  of  cell  life  already  described.1  We  have 
to  answer  this  question:  Are  the  nutrition  and  growth  of 
the  cell  and  the  formation  of  its  interior  and  exterior  proto- 
plasmic products  to  be  ascribed  to  the  cell-body,  or  does  the 
nucleus  participate  in  them  as  an  essential  element  ? 

B.  Hertwig  says,  in  his  '  Lehrbuch  der  Zoologie,'  7th  ed. 
p.  55  (Eng.  trans,  p.  67),  that  'for  a  long  time  the  functional  sig- 
nificance of  the  nucleus  in  the  cell  was  shrouded  in  complete 
darkness,  so  that  it  began  to  be  regarded,  in  comparison  with 
the  protoplasm,  as  a  thing  of  little  importance.'  In  fact,  a 
merely  superficial  consideration  of  the  phenomena  already 
described  might  easily  lead  us  to  doubt  any  participation  in 
them  on  the  part  of  the  nucleus.  If,  for  instance,  a  little 
Amoeba  grasps  its  still  smaller  prey  with  its  pseudopodia 
and  devours  it,  we  can  observe  a  series  of  movements 
about  and  in  the  viscous  protoplasm  of  the  creature's  body, 
but  we  can  perceive  no  change  in  its  nucleus.  If,  on  the  other 
hand,  a  plant  cell  is  trying  to  thicken  a  definite  portion  of  its 
enclosing  membrane  by  depositing  layers  of  cellulose,  the 
nucleus  may  be  seen  to  quit  its  former  position  in  the  centre 
of  the  cell,  and  to  approach  that  part  of  the  periphery  where 
the  depositing  action  of  the  protoplasm  is  at  its  height,  and, 
when  the  task  is  accomplished,  the  nucleus  comes  back  to  the 
middle  of  the  cell.  In  the  same  way  the  nuclei  of  certain 
unicellular  plant-hairs  approach  the  offshoot  as  long  as  it  is 
in  process  of  formation,  but  when  its  growth  is  complete  they 
return  to  their  original  place.  The  eggs  of  the  threadworm 
(Rhabdonema  nigrovenosum)  have  been  observed  during  the 
process  of  cleavage,  and  the  nuclei  of  the  newly  formed  cells 
moved  towards  the  surface  of  the  cell,  where  the  fresh  mem- 
brane was  forming,  and  after  remaining  there  for  some  time,  on 
the  completion  of  its  formation,  they  withdrew  into  the  centre 
of  the  cells.3 

1  Cf.   on  this  subject  especially   0.  Hertwig,  Allgemeine  Biologic.  (1906), 
chap.  10,  pp.  249,  &c. 

2  Cf.  L.    Rhumbler,    '  Uber    ein    eigentumliches    periodisches    Aufsteigen 
des  Kerns  an  die  Zelloberflache  innerhalb  der  Blastomeren  gewisser  Nematoden' 
(Anatomischer  Anzeiger,  XIX,  1901,  pp.  60-88).     See  also  the  address  delivered 
by  the  same  scientist  at  the  seventy-sixth  assembly  of  German  naturalists  at 
Breslau,  on  September  23,  1904,  and  printed  under  the  title  '  Zellenmechanik 
und  Zellenleben '  in  the  Naturwissenschaftliche  Rundschau,  1904,  Nos.  42  and 
43.     See  especially  pp.  546  and  548. 


Numerous  similar  phenomena,  pointing  to  a  participa- 
tion of  the  nucleus  in  the  processes  of  nutrition  and  forma- 
tion, were  described  in  1887  by  Haberlandt,  an  eminent 
botanist,1  and  in  1889  by  Korschelt,  a  zoologist.2  These  two 
scientists  deduced  the  following  conclusions  from  their 
observations  : — 

1.  The  fact  that  the  nucleus  occupies  a  definite  position 

only,  as  a  rule,  in  a  young  cell  in  course  of  development 
suggests  that  its  functions  are  connected  primarily 
with  the  processes  of  cell-development. 

2.  From  its  position  we  may  assume  that  the  nucleus  is 

especially  concerned,  during  the  growth  of  the  cell, 
with  the  thickening  and  spreading  of  the  cellular 
membrane  ;  but  it  is  quite  possible  that  in  a 
fully  grown  cell  the  nucleus  has  other  functions  to 

3.  The  nucleus  is  concerned  not  only  with  the  cell's  power 

of   secretion,   but   also   with  its   nutrition.     We   can 
infer   this    both    from    its    position    and    also    from 
the  fact  that  it  sends  out  numerous  branches,  thus 
increasing  its  surface  on  the  side  nearest  to  the  place 
where  secretion  or  nutrition  is  going  on.3 
We  must  refer  here  also  to  the  correlation  between  the 
size  of  the  protoplasmic  body  and  that  of  its  nucleus,  which 
E.  Hertwig  calls  the  Kernplasmar elation. ^    It  can  be  explained 
by  the  interior  reciprocal  action  of  the  cell-body  and  cell- 
nucleus.     What  actual  observation  pronounced  probable  has 
been  confirmed  by  experiments.     Gruber,  Nussbaum,  B.  Hofer, 
Verworn,  Balbiani,  Lillie,  Klebs   and  others  had  recourse  to 

1  '  t)ber  die  Beziehungen  zwischen  Funktion  und  Lage  des  Zellkerns  bei 
den  Pflanzen,'  Jena,  1887. 

2  '  Beitrage    zur  Morphologie  und  Physiologic    des    Zellkerns '     (Zoolog. 
Jahrbiicher,  Section  for  Anatomy,  IV,  1889). 

8  This  accounts  for  the  occurrence  of  nuclei  with  corners  or  even  branches 
in  the  gland-cells  of  certain  insects  when  in  a  state  of  active  secretion.  I 
noticed  such  nuclei  on  my  series  of  sections  of  the  ant-inquiline  Paussus 
cucullatus,  which  has  a  strongly  marked  layer  of  gland-cells  in  its  antennae. 
Similar  nuclei  occur  in  the  large  frontal  glands  which  open  through  an  exuda- 
tory  pore  of  the  forehead.  Cf.  '  Zur  Kenntnis  des  echten  Gastverhaltnisses 
bei  den  Ameisengasten  und  Termitengasten '  (Biolog.  Zentralblatt,  1903, 
pp.  240,  241,  244,  245).  .. 

4  Cf.  R.  Hertwig,  *  Uber  Korrelation  von  Zell-  und  Kerngrosse  fur  die 
geschlechtliche  Differenzierung  und  die  Teilung  der  Zelle '  (Biolog.  Zen- 
tralblatt, 1903,  Nos.  1  and  2).  See  also  0.  Hertwig,  Allgemeine  Biologie, 
p.  257. 


merotomy,  and  cut  unicellular  creatures  into  several  parts,1 
and  the  results  of  these  investigations  are  extremely  in- 

If  an  Amoeba  be  cut  into  several  pieces,  the  part  that  is 
fortunate  enough  to  contain  the  nucleus  continues  its  previous 
way  of  life  ;  it  moves  about  and  feeds,  and  so  it  replaces  what 
it  lost  in  living  substance  and  recovers  its  normal  size.  The 
other  parts,  however,  which  contain  no  nucleus,  soon  cease 
to  move,  and  in  course  of  time  the  network  of  protoplasm  that 
forms  their  body  begins  to  disintegrate,  until  nothing  is  left  of 
them.  A  non-nucleated  fragment  of  an  Amoeba  is  as  incapable 
of  feeding  as  it  is  of  moving.  It  can  no  longer  contract  so  as  to 
enclose  any  particle  of  nourishment  and  absorb  it  into  its  own 
body.  If  a  portion  of  an  Amoeba  had  already  begun  such  a 
nutritive  movement  before  its  separation  from  the  main  body, 
its  action  is  soon  arrested  and  the  inactivity  of  death  sets  in. 
In  the  case  of  unicellular  Khizopods,  which  deposit  a  chalky 
shell,  this  process  of  secretion,  being  analogous  to  the  formation 
of  membrane,  becomes  impossible  as  soon  as  the  nucleus  is 
removed,  but  the  nucleated  fragments  are  able  to  secrete 
a  shell  wherever  a  wound  has  been  inflicted. 

With  regard  to  plants,  too,  Klebs  has  shown  3  that  only  the 
nucleated  portions  of  a  plant  cell  are  able  to  form  a  new 
cellulose  membrane,  and  so  to  close  an  opening  cut  in  the 

Balbiani  has  succeeded  in  establishing,4  by  means  of 
merotomical  experiments  on  Infusoria,  the  precise  part  taken 
by  the  chromatin  of  the  nucleus  in  the  nutrition  and  growth 
of  unicellular  creatures.  In  a  previous  chapter  (pp.  60,  &c.) 
we  discussed  the  morphological  importance  of  chromatin  or 
nuclein  in  the  finer  structure  of  the  nucleus  ;  its  physiological 
importance  is  now  to  be  revealed. 

In  many  Infusoria  the  chromatin  is  arranged  in  numerous 

1  Merotomy  must  not  be  confused  with  merogony,  which  is  a  name  given 
to  attempts  to  fertilise  or  develop  ova  that  have  been  cut  up  or  otherwise 
artificially  mutilated.  We  shall  refer  to  this  subject  again  in  Chapter  VI,  §  8. 

*  Cf.  Wilson,  The  Cell,  pp.  342,  &c.     Also  0.  Hertwig,  pp.  254,  &c. 

3  Untersuchungen  ausdem  botanischen  Institutzu  Tubingen,  1888,  II,  p.  552. 

4  '  Recherches  experimentales  sur    la    merotomie    des    Infusoires    cilies  ' 
(Revue  Zoologique  Suisse,  V,   1889)  ;    '  Nouvelles  recherches  experimentales 
sur  la  merotomie  des  Infusoires  cilies  '  (Annales  d.  Micrographie,  IV,  1892 
and  V,  1893). 


somewhat  coarse  granules  in  the  interior  of  the  nucleus. 
Balbiani  succeeded  in  cutting  a  ciliated  Infusorian  (Stentor) 
into  three  pieces  in  such  a  way  that  the  nucleus  was  also  cut, 
each  segment  containing  a  part  of  it  (fig.  7). 

The  upper  division  containing  the  mouth  received  four 
granules  of  chromatin,  the  middle  portion  received  one,  and 
the  lowest  three.  All  three  parts  of  the  Stentor  continued  to 
live,  and  in  twenty-four  hours  each  had  become  a  fresh 
individual.  The  one  formed  from  the  middle  piece  of  the 

FIG.  7.— Stentor. 

On  the  left  (a)  is  the  specimen  cut  into  three  parts;  on  the  right  (b,  c,  d) 
the  new  specimens  formed  by  regeneration. 

k  =  nucleus  ;   v  =  vacuole. 

original  specimen  was,  however,  considerably  smaller  than 
the  other  two,  because  its  nucleus  had  possessed  only  one 
chromatin  granule. 

In  1896  Lillie  succeeded  in  dividing  a  Stentor  into  as  many 
pieces  as  he  wished,  by  simply  shaking  the  glass  vessel  con- 
taining it.1  In  this  way  he  was  able  to  show  that  fragments 
consisting  of  only  -^  of  the  creature's  volume  were  capable 
of  regeneration,  provided  they  contained  a  particle  of  the 
nucleus  ;  all  non-nucleated  portions  perished. 

In  other  merotomical  experiments  made  by  Balbiani,  the 
Infusorian  was  only  partially  severed,  so  that  the  two  parts 
remained  connected  by  the  protoplasm  of  the  cell-body.  If  the 

1  '  On  the  smallest  parts  of  Stentor  capable  of  regeneration '  (Journal  of 
Morphology,  XII,  Part  1). 



nucleus  was  not  cut,  the  wound  healed  quickly  and  the  creature 
recovered  its  previous  appearance  ;  it  never  happened  that 
two  individuals  were  formed  in  consequence  of  a  division  of 
this  kind.  If,  however,  the  nucleus  also  was  severed,  each 
part  of  the  Infusorian  grew  into  a  new  animal,  and,  as  they 
were  connected  by  a  piece  of  the  protoplasm,  the  result  of  this 
division  was  the  production  of  a  monstrous  double  creature 
that  reminds  one  of  the  famous  Siamese  twins.  In  course  of 
time,  however,  the  two  individuals  began  to  approach  one 
another,  their  nuclei  came  together  and  coalesced,  and  the 
monstrosity  became  one  normal  specimen. 

Other  experiments,  carried  on  by  Verworn  in  1891, l  and 
Balbiani  in  1892  and  1893,  have  led  to  a  modification  of  views 
based  on  the  experiments  just  described,  inasmuch  as  they 
have  thrown  additional  light  on  the  participation  of  the 
protoplasm  in  the  life  of  the  cell,  and  so  put  us  on  our_guard 
against  overrating  the  importance  of  the  nucleus.  Verworn 
cfiose  as  the  subject  of  his  experiments  a  spherical  Protozoon, 
Thalassicola,  which  measures  half  a  centimetre  across,  a 
gigantic  size  for  a  unicellular  creature.  He  succeeded  in 
isolating  the  nucleus  from  the  protoplasm  of  this  huge  cell- 
body,  and  demonstrated  unequivocally  that  the  nucleus  cannot 
live  alone  without  a  particle  of  protoplasm  ;  it  diecl  and  did 
not  lorm  a  new  cell- body  On  the  other  hand  the  non-nucleated 
cell-bodies  continued  alive  for  a^considerable  time  and  went 
on  feeding,  but  tney  were  unaEle  to  multiply  by  means  of 
division,  and  so  they  too  eventually  died.  In  his  more  recent 
experiments  Balbiani  compared  very  exactly  the  varying 
behaviour  of  nucleated  and  non-nucleated  portions  of  Infusoria. 
He  came  to  the  conclusion  that  nucleus  and  cytoplasm  are 
each  the  complement  of  the  other  in  discharging  the  most 
important  functions  of  life,  although  the  nucleus  plays  the 
chief  part.  Cytoplasm  alone  was  able  for  some  time  to  pro- 
duce the  movements  of  the  body  and  of  its  ciliated  envelope, 
the  ingestion  of  food  and  the  contraction  of  the  pulsating 
vacuoles  of  the  body.  The  nucleus  was,  however,  indispensable 
to  secretion,  regeneration,  and  the  processes  of  division,  without 
which  the  cell-plasm  must  inevitably  die. 

i  '  Die  physiologische  Bedeutung  des  Zellkerns  '  (Pfluger's  Archiv  fur  die 
gesamte  Physiologic,  LI). 


Not  only  zoologists,  but  also  botanists,  have  recently  been 
making  careful  experiments  with  a  view  to  determining  the 
part  taken  by  the  nucleus  and  the  cell-body  respectively  in 
the  vital  processes  of  the  cell.  The  results  show  that  in  plants 
too  the  value  of  the  cell-body  must  not  be  underestimated, 
although  the  nucleus  actually  controls  the  vital  activity  of 
the  cell.1 

I  have  already  (p.  80)  quoted  Klebs'  assertion  that  frag- 
ments of  vegetable  protoplasm  containing  no  nucleus  are 
incapable  of  forming  a  cellulose  membrane.  This  statement 
has  been  challenged  by  Palla  and  others,  who  think  that  they 
have  traced  the  formation  of  a  new  cell-wall  in  non-nucleated 
fragments,  although  other  botanists  regard  this  as  very 

Klebs  himself  mentions  the  fact  that  non-nucleated  frag- 
ments of  Algae  remained  alive  for  weeks,  but  eventually  died. 
I  may  therefore  on  this  point  agree  with  J.  Keinke,  the  botanist, 
when  he  says  :  3  '  The  nucleus  is  unquestionably  the  most 
important  organ  in  the  cell-body.' 

The  total  results  of  these  merotomical  experiments  may  be 
summed  up  shortly  as  follows  : — Nucleus  and  cytoplasm  are 
both  essential  to  the  life  of  a  cell.  A  cell-body  without  a 
nucleus  has  no  more  practical  value  than  a  nucleus  without 
a  body  of  protoplasm.  In  a  normal  cell  the  nucleus  is  to  a 
certain  extent  the  central  point,  the  organising  principle  of  the 
living  matter,  or,  as  Wilson  aptly  expresses  it,  '  the  controlling 
centre  of  cell-activity.'  4  Nevertheless,  after  the  nucleus  has 
been  removed,  the  cytoplasm  alone  is  in  many  cases  able  for 
a  time  to  continue  the  vital  processes  already  begun,  but  it 
is  incapable  of  producing  any  notable  new  formations,  and  is 
absolutely  unable  to  divide  and  to  perpetuate  the  species. 
The  nucleus  is,  as  will  be  shown  more  clearly  in  other  chapters, 
the  real  bearer  of  heredity,  and  within  the  nucleus  in  its  turn 
the  chromatin  is  chiefly  concerned  with  heredity. 

The  division  of  an  Infusorian  into  a  definite  number  of 
nucleated  pieces  results  in  the  formation  of  the  same  number 

1  Further  information  on  this  subject  will  be  found  in  Chapters  V  and  VI, 
where  I  shall  deal  with  cell-division  and  fertilisation. 

2  Cf.  Pfeffer,  Pflanzenphysiologie,  I  (1897),  pp.  45,  &c. 

3  Einleitung  in  die  theoretische  Biologic,  1901,  p.  256. 

4  The  Cell,  p.  30. 

o  2 


of  fresh  animals,  therefore  we  are  justified  in  calling  the  nucleus 
the  principle  of  individuation  of  living  matter  ;  and  here  again, 
within  the  nucleus,  it  is  to  the  chromatin  that  this  property 
must  especially  be  ascribed,  for  just  as  many  new  individuals 
are  formed  as  there  are  fragments  of  nucleus  containing 
chromosomes.  If  an  Infusorian  is  partially  severed,  a  double 
animal  is  formed  only  if  the  nucleus  be  cut  in  half. 

That  the  protoplasm  of  the  cell-body  is  not,  however, 
without  importance  in  the  formation  of  a  living  unit  seems 
to  be  proved  by  Balbiani's  experiment  with  the  double  Stentor. 
The  nuclei  of  the  two  creatures  gradually  approached  one 
another,  and  one  normal  animal  resulted  from  their  coalescence. 
If  there  had  been  no  living  bond  to  unite  them,  they  would  not 
have  grown  together  again  into  one  animal. 

Later  on  I  shall  have  to  discuss  the  important  part  played 
by  the  nucleus  and  its  chromatin  in  the  processes  of  cell- 
division  and  fertilisation.  In  this  place  I  may,  however, 
quote  a  passage  bearing  on  our  subject  from  K.  Hertwig's 
'Lehrbuch  der  Zoologie,'  1905,  p.  55  (English  translation, 
p.  67).  He  is  insisting  upon  the  significance  of  the  nucleus, 
and  says  :  *  The  evidence  that  the  nucleus  plays  the  most 
prominent  role  in  fertilisation  has  altered  this  conception 
(of  its  secondary  importance).  Then  arose  the  view  that 
the  nucleus  determines  the  character  of  the  cell ;  that 
the  potentiality  of-  the  protoplasm  is  influenced  by  the 
nucleus.  If  from  the  egg  a  definite  kind  of  animal  develop, 
if  a  cell  in  the  animal's  body  assume  a  definite  histological 
character,  we  are,  at  the  present  time,  inclined  to  ascribe 
this  to  the  nucleus.  From  this,  then,  it  follows  further  that 
the  nucleus  is  also  the  bearer  of  heredity  ;  for  the  transmission 
of  the  parental  characteristics  to  the  children  (a  fact  shown 
to  us  by  our  daily  experience)  can  only  be  accomplished 
through  the  sexual  cells  of  the  parents,  the  egg-  and  sperm- 
cells.  Again,  since  the  character  of  the  sexual  cells  is  deter- 
mined by  the  nucleus,  the  transmission  in  its  ultimate  analysis 
is  carried  on  by  the  nucleus.' l 

1  For  the  biological  and  physiological  importance  of  the  nucleus,  see  also 
Wilson,  The  Cell,  pp.  358,  359. 




Various  kinds  of  division  of  the  cell  (p.  86).  Various  kinds  of  division 
of  the  nucleus  (p.  87).  Direct  division  of  the  nucleus  (p.  87). 
Indirect  division  of  the  nucleus  (karyokinesis  or  mitosis)  (p.  88). 


Prophase  (spireme  or  monaster  stage)  (p.  90).  Metaphase  (the  chromo- 
somes split  lengthwise)  (p.  94).  Anaphase  (rearrangement  of  the 
chromosomes)  (p.  94).  Telophase  (dispireme  or  diaster  stage) 
(p.  95). 


The  part  played  by  the  centrosomes  (p.  98).  Debated  points  regarding 
their  importance,  occurrence,  and  origin  (p.  99).  Conclusions 
(p.  101). 

In  a  previous  section  (p.  66)  we  spoke  of  the  cells  as  the 
bricks  composing  the  building  of  the  organic  world.  But 
they  are  at-  the  same  time  the  architects,  always  rebuilding 
the  organic  world  in  an  unbroken  series  of  generations.  They 
are  living  constituents,  growing  and  multiplying  in  virtue  of 
the  laws  of  development  imposed  upon  them,  and  they  unite 
to  form  tissues,  organs,  and  living  creatures  of  various  kinds. 
The  fundamental  process  upon  which  the  architecture  of  the 
cell  depends  in  all  multicellular  organisms  is  that  of  cell- 
division.  What  the  delicate  scalpel  of  the  scientist  effects 
violently,  when  he  vivisects  unicellular  organisms  (see  p.  80), 
is  done  automatically  under  certain  circumstances,  in  accord- 
ance with  the  interior  laws  of  organic  growth  ;  and  one  cell, 
by  dividing,  forms  two  or  more. 

Let  us  now  study  this  natural  cell-division  and  the  interest- 
ing processes  that  attend  it. 


Whenever  the  development  of  an  individual  requires  an 
increase  in  the  number  of  cells,  whether  to  make  new  tissues, 
or  to  enlarge  those  already  existing,  or  to  form  new  creatures 



and  carry  on  the  process  of  propagating  the  species, — in  every 
case  the  cells  concerned  have  to  divide.  In  cells  containing  one 
nucleus,  the  first  step  is  the  division  of  the  nucleus.  Then 
the  protoplasm  of  the  cell-body  either  divides  too,  or  remains 
undivided  ; l  in  the  latter  case  a  uninuclear  cell  becomes  multi- 
nuclear  ;  in  the  former,  which  is  much  more  common,  one  cell 
becomes  several.  If  the  cellular  membrane  is  divided  and 
fresh  cell-walls  are  formed,  we  have  exogenous  cell-division  ; 
but  if  the  daughter-cells  remain  within  the  membranous 
covering  of  the  mother-cell,  we  have  what  is  called  endogenous 
cell-division.2  When  exogenous  cell-division  takes  place, 
the  new  cells  either  remain  side  by  side,  so  that  a  cellular 
tissue  is  formed,  or  they  leave  their  homes  and  migrate. 
Again,  when  a  cell  divides,  it  may  form  two  or  more  cells  of 
equal  size,  and  this  is  simple  cell-division  ;  or  the  new  cells 
cut  off  from  the  mother-cell  may  be  much  smaller  than  it  is  ; 
this  kind  of  division  is  called  gemmation — it  occurs  in  the  growth 
and  multiplication  of  many  of  the  lower  animals,  for  instance, 
in  the  Podophrya,  the  Hydra,  &c.,  and  in  some  plants,  such  as 
the  yeast  fungus.  Whatever  be  the  form  of  cell-division,  its 
chief  feature  is  invariably  the  division  of  the  nucleus,  and  we 
must  therefore  devote  attention  particularly  to  it.  We 
here  touch  upon  a  subject  with  regard  to  which  modern  micro- 
scopical research  has  been  most  successful ;  in  fact,  it  would 
be  difficult  to  name  any  other  subject  in  dealing  with  which 
microscopical  research  has  produced  more  brilliant  results, 
so  great  have  been  the  delicacy  and  intelligence  with  which 
the  investigations  have  been  conducted,  and  so  bold  and 
shrewd  the  conclusions  deduced  from  their  results,  although 
these  conclusions  are  to  a  large  extent  still  hypothetical. 
Modern  cytology  has  succeeded  in  some  degree  in  solving  the 
mysteries  of  heredity,  by  means  of  microscopical  research. 
If  we  are  careful  to  distinguish  the  actual  results  from 
the  conclusions  deduced  from  them,  we  shall  be  able 

1  The  process  of  division  which  affects  only  the  nucleus  and  does  not  result 
in  a  cell-division  is  sometimes  called  '  free  nuclear  division.'     (Cf.  Strasburger, 
Lehrbuch  der  Botanik,  1895,  pp.  55,  &c.     Eng.  trans.  1893,  pp.  89,  90.)     This 
free  nuclear  division  must  not  be  confused  with  '  free  formation  of  the  nucleus,' 
to  which  I  shall  refer  later. 

2  On  the  subject  of  endogenous  increase  of  nuclei,  resulting  in  the  presence 
of  several  nuclei  in  one   cell,    see    0,    Hertwig,    Allgemeine  Biologie,    1906, 
pp.  213,  &q, 


subsequently  to  form  a  true  opinion  of  the  modern  theories  of 

Nuclear  division  is  either  direct  or  indirect.  In  the  former, 
the  division  of  the  nucleus  takes  place  without  causing  any 
essential  change  in  its  structure  ;  but  in  the  latter  it  is  accom- 
panied by  a  complicated  mechanism,  involving  great  changes 
in  the  structure  of  the  nucleus,  and  partially  also  in  the  proto- 
plasm of  the  cell.  These  changes  are  chiefly  in  the  position 
and  arrangement  of  the  chromatin  constituents  of  the  nucleus, 
viz.  the  nuclear  thread  and  its  chromosomes  ;  but  there  are 
also  no  less  regular  formations  of  fibres  and  asters  out  of  the 
achromatic  nuclear  substance. 

On  account  of  the  characteristic  movements  of  the  chromatin 
in  the  nucleus,  the  indirect  nuclear  division  is  sometimes  called 
karyokinesis  (nuclear  movement),  while  the  transformation 

FIG.  8. — Direct  division  of  the  nucleus  in  red  blood-corpuscles. 

and  breaking  up  of  the  chromatin  thread  and  the  simultaneous 
appearance  of  achromatic  spindle  fibrils  have  given  rise  to  the 
name  mitosis  (/uro?  =  thread)  or  mitotic  division,  whereas  the 
direct  division  is  called  amitotic.  Let  us  begin  by  considering 
the  latter,  as  it  is  the  simpler  form,  and  will  help  us  to  under- 
stand the  more  complex  process  of  indirect  division. 

Direct  division  of  the  nucleus  was  observed  by  Remak  in 
red  blood-corpuscles  as  early  as  1841.  Young  corpuscles 
contain  one  nucleus,  the  division  of  which  leads  to  their  multi- 
plication. The  process  is  very  simple,  as  the  accompanying 
figure  will  show. 

The  nucleus  in  the  cell  is  at  first  spherical,  then  it  elongates, 
gradually  contracting  in  the  middle.  At  the  same  time  the 
cell  itself  assumes  an  oval  shape,  having  previously  been 
round.  The  nucleus  next  splits  in  half,  and  the  two  halves 
retire  from  one  another  ;  then  the  protoplasm  of  the  cell-body 
contracts  in  the  middle,  the  indentation  deepening  until  finally 
two  spherical  blood-cells  are  formed,  each  with  a  round  nucleus 


in  its  centre.  Therefore,  in  the  course  of  direct  cell-division, 
the  nucleus  by  simply  contracting  breaks  into  two,  and  then 
the  protoplasm  of  the  cell-body  and  the  cellular  membrane 
divide  likewise.  This  form  of  division  of  the  nucleus  and  cell 
occurs  frequently  among  Protozoa,  especially  among  those 
possessing  a  nucleus  that  is  rich  in  chromatin. 

There  is  some  uncertainty  as  to  the  discoverer  of  indirect 
division.  Wilson  ('  The  Cell,'  p.  64)  ascribes  the  discovery  of 
mitosis  to  Anton  Schneider,  a  zoologist,  in  1873.  Sachs  thinks 
J.  Tschistiakoff,1  a  botanist,  has  a  better  claim  to  the  honour, 
as  his  work,  published  in  1874,  gave  the  first  impulse  to  modern 
research  on  this  subject.  Others  again  mention  E.  Strasburger, 
the  botanist,  as  the  discoverer  of  this  complicated  form  of 
cell-division.  There  is  no  doubt  that  the  German  anatomist, 
Walter  Flemming,  was  the  first  to  formulate  and  expound  the 
process  of  mitosis  in  his  '  Beitrage  zur  Kenntnis  der  Zelle 
und  ihre  Lebenserscheinungen  '  (1 878-82). 3  Abbe  Carnoy,  a 
Belgian,  has  thrown  much  light  upon  the  subject  in  his 
'  Biologie  cellulaire  '  (1884),  and  by  means  of  his  admirable 
study  of  cell-division  in  Arthropods.3 

It  would  be  superfluous  to  mention  more  names,  for  the 
study  of  mitosis  has  now  become  a  favourite  branch  of  cyto- 
logical  research,  and  we  know  that,  in  the  case  of  very  different 
kinds  of  tissue,  indirect  division  of  the  nucleus  occurs  far  more 
generally  than  direct.  The  two  great  forms  of  division  of 
nucleus  and  cell  are,  however,  connected  by  various  inter- 
mediate forms. 

A  very  thorough  discussion  of  all  the  phenomena  observed 
in  mitosis  may  be  found  in  Wilson's  '  The  Cell/  pp.  65-121,  a 
book  that  I  have  frequently  had  occasion  to  mention.  My 
own  account  of  the  process  must  be  limited  to  the  barest  outlines. 


We  have  seen  that  in  direct  division  of  the  nucleus,  or 
amitosis,  the  division  of  the  chromatin  elements  of  the  nucleus 

1  Sachs,  Vorlesungen  fiber  Pflanzenphysiologie,  1887,  p.  115,  note  4.     Tschi- 
stiakoff's  work  to  which  Sachs  refers  is  his  '  Materiaux  pour  servir  a  1'histoire 
de  la  cellule  vegetale  '    (Nuovo  Giornale  Botan.   Ital.  VI).     See   particularly 
Plate  VII,  figs.  11-13. 

2  Archiv  Jur  mikroskopische  Anatomie,  XVI-XIX. 

8  '  La  Cytodierese  chez  les  Arthropodes  '  (La  Cellule,  I,  1885,  No.  2). 


in  the  mother-cell,  so  as  to  form  the  nuclei  of  the  two  daughter- 
cells,  is  effected  by  means  of  a  rough  partition  of  the  mother- 
nucleus,  which  first  contracts  in  the  centre  and  then  splits 
in  half.  In  indirect  cell-division,  or  mitosis,  there  is  a 
complicated  series  of  phenomena,  all  aiming  at  dividing  the 
chromatin  of  the  mother-nucleus  in  a  most  exact  and  regular 
fashion  between  the  two  daughter-nuclei.  This  may  be  called 
the  fundamental  idea  underlying  the  whole  process  of  karyo- 
kinesis  or  mitosis,  and  all  the  other  incidents  are  subordinate 
to  it. 

It  is,  however,  as  E.  B.  Wilson  rightly  remarks,  difficult  to 
give  a  connected  general  account  of  mitosis,  because  the  details 
vary  in  many  respects  in  different  cases,  and  especially  because 
great  uncertainty  still  hangs  over  the  nature  and  functions  of 
the  so-called  centrosome.  In  German  textbooks  of  zoology 
we  generally  find  the  process  of  karyokinesis  exemplified  by 
the  nuclear  divisions  of  the  epithelial  cells  of  the  spotted 
salamander  (Salamandra  maculosa),  and  my  own  experience 
shows  that  these  supply  us  with  an  excellent  means  of  tracing 
the  process  of  karyokinesis  conveniently.  It  is  only  necessary 
to  cut  off  a  piece  of  the  epidermis  from  the  tail  of  a  salamander 
or  triton  larva,  to  treat  it  in  the  usual  way  with  carmine  or 
haematoxylin,  so  as  to  prepare  it  for  the  microscope,  and  then 
it  is  possible  to  see  a  series  of  karyokinetic  figures  in  the  cells 
of  the  epithelium.  In  order  to  be  able  to  distinguish  the 
single  chromosomes,  we  generally  have  recourse  to  some 
special  staining  methods,  and  Heidenhain's  stain  with  iron- 
haematoxylin  can  still  be  recommended.  In  discussing  the 
subject,  however,  I  shall  refrain  from  alluding  to  differences  in 
single  instances  and  in  staining  methods,  and  shall  follow 
Wilson's  admirable  account  of  karyokinesis  in  '  The  Cell,' 
pp.  65-72. 

We  may  distinguish  four  groups  of  phenomena  as  four 
successive  stages  in  karyokinesis.  There  are  : — (1)  the 
Prophase  or  preparatory  changes ;  (2)  the  Mesophase  or 
Metaphase,  in  which  the  chromatin  substance  of  the  nucleus  is 
actually  divided  ;  (3)  the  Anaphase,  in  which  the  divided 
nuclear  elements  are  rearranged  so  as  to  form  the  daughter- 
nuclei  ;  (4)  the  Telophase,  in  which  the  cell  finally  divides 
and  the  daughter-nuclei  return  to  the  state  of  rest. 


These  four  stages  are,  of  course,  not  sharply  marked  off 
from  one  another,  but  one  gradually  passes  into  another. 

In  all  four  we  see  a  double  series  of  changes  going  on 
simultaneously  in  the  cell.  The  first  involves  the  chromatin 
figures  of  the  nucleus,  formed  by  the  change  in  position  and 
the  halving  of  the  chromatin  substance  of  the  nucleus  ;  the 
second  series  involves  the  achromatic  nuclear  figures,  resulting 
from  changes  in  the  achromatic  nuclear  framework,  and  to 
some  extent  also  from  changes  in  the  achromatic  cell-frame- 
work. The  first  series  of  changes  effects  the  actual  division 
of  the  nucleus  ;  the  second  series  is  subsidiary,  and  consists  of 
a  radiating  arrangement  of  the  protoplasm,  rendering  possible 
the  movements  that  occur  in  the  first  series. 

Let  us  now  examine  some  diagrams  (figs.  9-16)  which 
will  give  us  a  better  idea  of  the  marvellous  mechanism  of 

1.  Prophase. — The  first  step  towards  indirect  division  of  the 
nucleus  is  a  change  in  the  chromatin  substance.  When  the 
cell  was  resting,  this  appeared  as  a  coil  of  thread  or  as  a  reticular 
or  alveolar  framework,  but  now  it  thickens  into  a  skein.  Fig.  9 
represents  a  cell  at  rest,  with  its  reticular  chromatin  frame- 
work of  the  nucleus.  The  dark  spot  n  within  the  network  is 
a  nucleolus  (see  pp.  54  and  61),  but  its  presence  is  not 
essential ;  c  is  the  centrosome  already  in  process  of  division — 
it  is  a  spherical  body,  only  slightly  susceptible  to  stains,  which 
is  also  called  the  polar  body,  from  its  position.  Boveri  terms 
it  the  organ  of  cell-division,  and  he  is  probably  right  in  so 
doing,  as  we  shall  see  later.1 

In  Fig.  10  the  prophase  of  karyokinesis  has  begun,  and 
the  chromatin  thread  of  the  nucleus  has  thickened  and  con- 
tracted, so  as  to  form  one  unbroken  skein.  The  nucleolus  n  is 
still  visible,  the  centrosome  has  divided,  so  that  there  are  now 
two,  which  are  moving  apart  and  beginning  to  send  out  delicate 
rays  of  protoplasm  to  form  the  attraction-sphere  a.  This  is 
sometimes  called  the  chromatin  skein  or  spireme  stage  of  cell- 
division,  from  the  arrangement  of  the  chromatin  substance  of 
the  nucleus.  As  it  often  forms  a  kind  of  rosette,  it  has  also 
been  described  as  the  chromatin  monaster  (single  star)  stage. 

1  This  polar  body  must  not  be  confused  with  the  directing  or  polar  globule 
of  the  egg-cell.  See  Chapter  VI,  §  2. 



Lastly,  as  the  achromatic  centrosome  figure  (a  in  fig.  10) 
resembles  a  double  star,  it  is  sometimes  called  the  achromatic 
amphiaster  stage.  The  farther  apart  the  two  centrosomes 
move  in  order  to  take  up  their  position  at  the  opposite  poles 

FIG.  9. 

FIG.  10. 

FIG.  11. 

FIG.  12. 

FIG.  9. — Cell  with  resting  nucleus. 
FIGS.  10-12. — Prophases  of  mitosis  (Wilson). 

c  —  centrosome ;   n  =  nucleolus ;    a  —  amphiaster ;   sp  =  spindle ; 
chr  =  chromosomes  ;   aek  =  equatorial  plate. 

of  the  nucleus,  the  more  applicable  becomes  the  name  amphi- 
aster to  this  achromatic  figure. 

Fig.  11  represents  the  second  stage  of  prophase.  The 
double  star  or  amphiaster  now  forms  an  achromatic  spindle, 
and  the  chromatin  figure  shows  remarkable  changes.  The 


chromatin  spireme  thread  has  broken  up  into  a  number  of 
regular  segments,  which  form  the  chromosomes.  They 
originally  composed  the  chromatin  network  of  the  nucleus, 
and  at  each  cell-division  they  appear  in  the  same  shape  and 

Tlje  chromosomes  of  the  same  nucleus  are  generally  all  of 
the  same  size  and  shape,  but  occasionally  they  form  a  series 
of  pairs,  and  in  some  very  rare  cases  superfluous  or  accessory 
chromosomes  appear.  They  have,  as  a  rule,  the  shape  of  a 
fairly  regular  U  or  V,  sometimes  however  they  are  rod -like  or 
even  spherical.  In  certain  cases  the  lengthwise  division  of  the 
chromosomes,  which  takes  place  in  the  metaphase,  is  suggested 
previously,  as  each  splits  lengthwise  into  two  parallel  parts, 
which  remain  connected  by  delicate  transverse  fibres.  (See 
the  chromosomes  in  fig.  11.) 

As  we  shall  see  in  the  next  chapter,  the  chromosomes 
are  of  very  great  importance  in  the  propagation  of  the  race 
and  in  the  transmission  of  hereditary  characteristics,  and 
therefore  we  must  devote  a  little  more  attention  to  them. 
In  all  plants  and  animals  propagated  by  the  union  of  two  sexes, 
the  number  of  chromosomes  in  every  cell  is  invariably  even, 
one  half  being  derived  from  each  of  the  parents.  Further, 
with  very  few  exceptions,  every  species  of  plant  and  animal 
has  always  the  same  fixed  number  of  chromosomes  in  every 

Only  the  germ-cells  are  an  important  class  of  exceptions, 
as  we  shall  see  in  the  next  chapter,  for  they  contain  only  half 
as  many  chromosomes  as  the  other  cells  of  the  body. 

The  number  of  chromosomes  in  each  cell  varies  very  greatly 
in  different  species  of  animals  and  plants.  It  ranges  from  2 
to  168.  Sometimes  there  is  a  considerable  difference  in  the 
number  of  chromosomes  of  closely  related  species,  whilst 
on  the  other  hand  those  of  unconnected  species  are  often 
identical  in  number.  Any  one  who  is  interested  in  the  subject 
may  find  the  chromosome  numbers  of  sixty-two  species  of 

1  Boveri  has  based  his  theory  of  the  individuality  of  chromosomes  upon 
this  fact.     See  Chapter  VI,  §  9. 

2  The  threadworm,  Ascaris  megaJocephala,  has  two  varieties,  one  of  which 
contains  four,  and  the  other  two,  chromosomes  in  the  cells  of  its  body.     For 
other  instances  see  Korschelt  and  Heider,    '  Lehrbuch     der   vergleichenden 
Entwicklungsgeschichte  der  wirbellosen  Tiere  '  (Allgem.  Teil,  part  2,  p.  612). 


plants  and  animals  tabulated  on  p.  206  of  Wilson's  '  The 
Cell.'  i 

I  quote  from  it  a  few  numbers  by  way  of  example  ;  they 
are  those  of  the  chromosomes  in  the  somatic  cells  of  each 
species  ;  in  the  ripe  germ-cells,  as  has  been  said  before,  only 
half  the  number  of  chromosomes  occurs. 

In  many  worms  there  are  2  or  4  chromosomes ;  in 
others  8  ;  in  some  Medusae,  grasshoppers  and  Phanerogams, 
12 ;  in  one  Hydrophilus,  a  snail,  the  ox  and  man,  16  ;  in 
the  sea-urchin  and  a  sea -worm  (Sagitta),  18  ;  in  an  ant  (Lasius), 
20  ;  in  the  lily,  the  salmon,  the  frog  and  the  mouse,  24  ;  in  the 
torpedo,  36 ;  in  a  worm  (Ascaris  lumbricoides),  48 ;  and  in  a 
little  fresh -water  crab  (Artemia),  168. 

Let  us  now  turn  to  fig.  11,  and  follow  the  movements  of  the 
chromosomes  during  karyokinesis.  We  see  that  the  chromatin 
within  the  nucleus  now  appears  as  an  independent  formation. 
The  nuclear  membrane  enclosing  the  nucleus  has  meantime 
disappeared,  and  so  has  the  nucleolus  (n  in  figs.  9  and  10). 2 

The  two  centrosomes,  which  in  fig.  10  are  still  above  the 
nucleus,  have  now  taken  up  their  position  at  its  two  poles. 
The  protoplasmic  rays  proceeding  from  them  have  grown 
longer,  and  now  meet  in  the  centre  of  the  nucleus  forming  the 
nuclear  spindle  (sp).  This  is  also  called  the  direction  spindle, 
because  it  serves  to  direct  the  chromosomes  in  their  movement 
both  before  and  after  the  actual  division.  The  chromosomes 
now  lie  apparently  free  in  the  middle  of  the  cell,  but  in  reality 
they  are  connected  with  the  fibres  of  the  achromatic  spindle, 
which  are,  as  a  rule,  formed  out  of  what  was  previously  the 
achromatic  nuclear  framework,  but  in  some  cases  out  of  the 
cell  framework,  or  out  of  both  together.3 

This  stage  (fig.  11)  is  called,  from  the  chromatin  nuclear 
figure,  the  stage  of  chromatin  loops,  or,  from  the  achromatic 
figure,  the  stage  of  the  direction  spindle. 

1  Of.  also  0.  Hertwig's  Allgemeine  Biologie,  1906,  p.  203,  where  the  same 
table  is  given  with  some  additions. 

2  On  the  behaviour  of  nucleoli  in  different  cases,  see  Wilson,  The  Cell, 
pp.  67,  68. 

3  There  was  for  a  long  time  great  divergency  of  opinion  regarding  the 
origin  of  the  protoplasmic  spindle-fibres.     Modern  research  seems  to  show 
that  we  ought  to  distinguish  three  kinds  of  spindle  :   (a)  those  that  are  formed 
of  the  nucleus  alone ;    (6)  those  that  are  formed  of  the  cell  cytoplasm ;    and 
(c)  those  that  are  of  mixed  origin.     Cf.  0.  Hertwig,  Allgemeine  Biologie,  1906, 
pp.  193-195. 


Fig.  12  depicts  the  third  part  of  the  prophase,  which 
leads  on  to  the  metaphase.  The  chromosomes  are  moving 
along  the  spindle-fibres  towards  the  centre,  and  finally  group 
themselves  in  the  form  of  a  ring  in  a  plane  passing  through 
the  equator  of  the  spindle,  which  is  known  as  the  equatorial 

From  the  chromatin  nuclear  figure,  this  stage  is  called  that 
of  the  equatorial  plate,  or  rather  crown  (aek  in  fig.  12),  because 
the  chromosomes  remain  distinct  from  one  another,  and  only 
group  themselves  in  the  shape  of  a  ring.  The  achromatic 
nuclear  figure,  the  spindle  (sp),  is  best  seen  in  this  stage. 

2.  Metaphase. — The    middle    stage,    or    metaphase,    now 
begins,  and  is  the  culminating  point  of  the  whole  karyokinesis, 
because  in  it  the  actual  division  of  the  nucleus  takes  place 
(fig.  13).     In  1880  W.  Flemming  discovered  that  this  division 
consists  of  the  splitting  of  the  chromosomes  lengthwise  into  two 
exactly  similar  halves.     If  each  chromosome  had  originally  the 
shape  of  a  V,  it  now  becomes  a  W  ;  if  it  was  a  simple  rod,  it  is 
now  a  double  one.     This  division  of  the  chromatin  nuclear 
substance   takes   place  with   such    extraordinary   exactitude, 
that  it  is  impossible  to  avoid  regarding  it  as  of  great  importance 
to  the  processes  affecting  heredity.     As  W.  Eoux  showed  in 
1883,  the  entire  chromatin  of  the  nucleus  in  the  mother-cell  is 
divided  according  to  the  strictest  rules  of  distributive  justice, 
so  that  the  nuclei  of  the  daughter-cells  receive  precisely  equiva- 
lent portions,  and  each  portion  is  arranged  in  exactly  the  same 
number  of  chromosomes   as  there  were   in  the   mother-cell. 
It  is  a  matter  of  indifference  whether  the  lengthwise  splitting 
of  the  chromosomes  in  the  metaphase  was  anticipated  by  a 
longitudinal  division  of  each  single  chromosome  (fig.  11),  or 
whether  the  whole  process  takes  place  at  once.     The  nucleolus  n 
may  remain  visible  during  the  metaphase  (as  in  fig.  13)  or  it 
may  disappear.     Its  behaviour  is  of  minor  importance. 

This  central  stage  of  indirect  cell- division,  which  we  have 
just  described,  is  known  as  the  stage  of  doubling  the  equatorial 

3.  Anaphase. — In   this   stage   the   daughter-nuclei   of   the 

1  For  the  sake  of  simplicity,  the  chromosomes  on  the  diagram  are  repre- 
sented as  rod-like  rather  than  curved,  although  the  latter  is  the  more  usual 
form.  Each  loop  points  to  the  centre  of  the  equatorial  plate. 



new  cells  are  built  up.  After  splitting  lengthwise  in  the 
metaphase  (fig.  13),  the  two  halves  of  each  chromosome 
begin  to  draw  apart.  Those  on  the  right  group  themselves 
about  the  right  pole  of  the  spindle,  and  those  on  the  left  about 
the  left  pole,  the  spindle-fibres  serving  as  guides.  Fig.  14 

FIG.  13. — Stage  of  metaphase. 

FIG.  14. — Stage  of  anaphase. 

FIGS.   15  AND  16. — Stages  of  telophase  (Wilson). 

c  =  centrosome ; 
ep  =  equatorial  plate ; 

n  =  nucleolus  ; 
pk  =  polar  caps  ; 

if  =  interzonal  fibres  ; 
zp  =  cell-plate. 

represents  this  stage  of  the  anaphase.     It  is  known  as  that  of 
dicentric  orientation  of  the  daughter-chromosomes. 

4.  Telophase. — The  process  of  karyokinesis  now  advances 
rapidly  through  its  final  stages  or  telophase.  Fig.  15  represents 
the  transition  from  the  anaphase  to  the  telophase.  The 
chromosomes  of  the  daughter-nuclei  have  now  reached  the 
two  opposite  poles  of  the  spindle,  have  grouped  themselves 
together  and  sent  out  delicate  fibres,  which  bind  them  together 


and    will    eventually  enable    them    to    unite    and    form   the 
chroma  tin    framework     of    the     daughter-nuclei.     In    some 
cases  the  chromosomes  do  not  directly  coalesce  to  form  the 
new  nuclear  framework,  but  it  is  produced  by  the  fusion  of 
vesicles  to  which  the  chromosomes  have  given  rise  (vacuoli- 
sation).1     From  the  chromatin  nuclear  figure,  which  forms  a 
dark  coloured  ring  round  the  two  poles  of  the  cell  in  course  of 
division  (fig.  15),  this  stage  has  been  called  that  of  the  two 
polar   caps   or   crowns.     If  these   crowns   assume   a   stellate 
shape,  it  is  called  the  stage  of  the  chromatin  diaster  or  double 
star.     When,  as  in  the  epithelial  cells  of  Amphibia,  the  egg- 
cells  of  Ascaris  and  many  plant  cells,  the  chromatin  framework 
of  the  new  daughter-cells  is  not  produced  by  vacuolisation  of 
the  chromosomes,  but  by  their  thickening  and  growing  together, 
the  chromatin  diaster  stage  is  followed  immediately  by  that 
of  the  chromalin  dispireme.    We  can  form  some  idea  of  this,  if 
we  imagine  the  ends  of  the  chromosomes  within  the  future 
daughter-cells   in  fig.  15  to  be   united.     This  would  produce 
two  skeins  similar  to  that  which  we  noticed  in  the  prophase 
(fig.  10)  as  the  beginning  of  the  division  of  the  chromosomes. 
The  fibres  of  the  spindle,  which  appear  in  fig.  15  uniting 
the  two  chromatin  asters,  have  now  another  name.     They 
are  called  interzonal  or  connecting  fibres  (if).     In  almost  all 
plant  cells,  and  occasionally  in  animal  cells,  they  are  thickened 
in  the   middle,   and   these   thickened   portions   subsequently 
make  up  the  cell-plate  (zp)  or  mid-body  of  the  dividing  cells. 
At  the  end  of  the  telophase  we  reach  the  last  stage  of 
indirect  division  of  the  nucleus  (fig.  16).     The  two  chromatin 
skeins   of  the   daughter-nuclei  have   surrounded   themselves 
with  a  membrane,  within  which  the  new  framework  has  been 
formed.     We   can   again  perceive   the   nucleolus    (n)   in   the 
nucleus.     Each    daughter-nucleus    has    brought    with    it    a 
centrosome  into  the  new  cell,  where  it  will  divide,  and  the 
two  fresh  centrosomes  will  move  from  the  poles  to  the  two 
sides  of  the  equator  of  the  original   karyokinetic  figure  and 
take  up  their  position  there.     This  is,  however,  not  always 
the  case.     Sometimes  they  vanish  altogether,  and  reappear 
only  when  the  process  of  division  is  to  begin  again.     The  fate 

1  For  further  information  regarding  the  growth  of  the  nucleus,  see  Wilson, 
The  Cell,  p.  71. 


of  the  interzonal  fibres  (if),  which  remind  us  of  the  spindle 
of  the  former  achromatic  karyokinetic  figure,  varies  greatly. 
In  plant  cells  they  remain,  and  by  thickening  they  help  to 
build  up  the  new  cell-walls  formed  by  the  secretion  of  cellulose.1 
Fig.  16  gives  us  an  instance  of  this.  The  perpendicular  line 
in  the  middle  represents  the  cell-plate  (zp)  or  mid-body  of 
the  cell  in  course  of  division.  In  animal  cells,  on  the  contrary, 
the  interzonal  fibres  generally  disappear  early  and  no  trace 
of  them  remains,  as  they  are  not  in  this  case  needed  to  form 
a  cell-plate.  Fig.  15  shows  the  mother-cell  with  deep  indenta- 
tions above  and  below  ;  these  increase  until  it  finally  splits 
in  half,  and  the  two  daughter-cells  are  formed,  and  thus 
the  process  of  indirect  division  of  the  nucleus  and  cell  is 


Let  us  review  once  more  the  phenomena  of  karyokinesis. 
The  first  two  stages  of  the  prophase,  those,  namely,  of  the 
chromatin  spireme  and  the  chromatin  monaster,  correspond 
exactly  to  the  last  two  stages  of  the  telophase,  those  of  the 
chromatin  diaster  and  the  chromatin  dispireme.  The  stages 
lying  between  these  two  extremes  belong  to  the  doubling  of  the 
equatorial  plate  or  crown.  This  culminating  point  is  connected 
on  the  one  hand  with  the  prophase,  by  the  breaking  up  of  the 
chromatin  monaster  into  V-shaped  segments,  and  by  their  group- 
ing to  form  a  simple  equatorial  plate  ;  it  is  connected  on  the 
other  hand  with  the  anaphase,  by  the  dicentric  orientation  of  the 
daughter-segments  in  the  double  equatorial  plate,  and  with  the 
telophase  by  their  withdrawal  to  the  poles  and  formation  of 
the  two  polar  caps  or  crowns.  Indirect  karyokinesis  is  there- 
fore a  process  that  is  at  once  marvellously  complex  in  its 
conformity  to  law,  and  wonderfully  simple  in  design.  Its 
object  is  to  divide  the  chromatin  of  the  nucleus  in  the  mother- 
cell  into  two  absolutely  equal  parts,  in  such  a  way  that  the 
nucleus  of  each  of  the  two  daughter-cells  shall  receive  the  half 
of  every  chromosome  in  the  mother-cell,  and  that  the  number 
of  chromosomes  in  each  daughter-nucleus  shall  be  the  same 
as  that  of  the  chromosomes  in  the  mother-nucleus. 

1  Cf  Strasburger,  Lehrbuch  der  Botanik,  1895,  p.  52. 


The  account  just  given  of  indirect  karyokinesis  and  the 
diagrams  illustrating  it  must  be  regarded  as  in  some  degree 
theoretical,  for  many  modifications  occur  in  various  kinds  of 
animals  and  plants.1 

Reinke  says  very  truly  in  his  i  Einleitung  in  die  theoretische 
Biologie,'  p.  260  :  '  To  variations  in  the  structure  of  the  nucleus 
in  different  organisms  correspond  variations  in  the  course  of 
mitosis,  as  will  be  seen  by  comparing  them.  But  we  find  every- 
where four  fundamental  phenomena,  viz.  the  formation  of  the 
chromatin  and  achromatic  figures  out  of  the  resting  nucleus  ; 
the  splitting  of  the  chromosomes  ;  the  movement  of  the 
divided  chromosomes  to  the  poles  of  the  mitotic  figure  ;  and 
the  rearrangement  of  the  parts  so  as  to  reproduce  the  configura- 
tion of  the  resting  nucleus.  The  persistence  of  the  number 
of  chromosomes  from  generation  to  generation  in  nuclei  of 
the  same  species  may  be  added  as  a  fifth  point.' 

The  polar  bodies  called  centrosomes  were  discovered  by 
Flemming  in  1875,2  and  I  have  designated  them  and  the 
spindle  radiating  from  them  a  biomechanical  contrivance  for 
securing  a  regular  division  of  the  chromatin.  This  view  is 
confirmed  by  the  account  of  karyokinesis  given  by  the  best 
authors.  We  may  therefore  follow  Boveri,  Weismann,  and 
others  in  calling  the  centrosomes  the  especial  organs  of  cell- 

E.  Bergh  is  inclined  to  ascribe  even  greater  importance  in 
the  process  of  cell-division  to  the  achromatic  than  to  the 
chromatin  nuclear  figure.4  E.  "van  Beneden,  Flemming, 
Guignard  and  others  are.  also,  perhaps,  disposed  to  overrate 
the  importance  of  the  centrosomes.5 

1  This  is  true  of  the   normal  processes   concerned   in   karyokinesis,    but 
there  are  other  modifications  which  are  matters  of  pathology,  and  which 
we  cannot  discuss  here.     See  0.  Hertwig,  Allgemeine  Biologie,  pp.  214,  &c. 

2  On  the  subject  of  centrosomes  see   0.   Hertwig,   Allgemeine  Biologie, 
pp.  45-49,  195,  &c.,  and  E.  B.  Wilson,  The  Cell,  pp.  50,  &c.,  74,  &c.,  101,  &c., 
208,  &c.,  354,  &c.  ' 

3  In  the  next  chapter  we  shall  have  to  examine  Boveri's  opinion  regarding 
the  importance  of  the  centrosomes  as  fertilising  elements.     Cf.  also  Boveri, 
Zellenstudien,  Part  4.      'Uber  die  Natur  der   Centrosomen'  (Jenaische  Zeit- 
schrift  fur  Naturwissenschaft,  1901). 

4  'Kritik  einer  modernen  Hypothese  von  der  Ubertragungerblicher  Eigen- 
schaften  '  (Zoologischer  Anzeiger,  XV,  1892,  No.  383). 

5  See  also  V.  Haecker,   '  Uber  den  heutigen  Stand  der  Centrosomenfrage ' 
(Verhandl.  der  Deutschen  Zoologischen  Gesellschaft,  1894,  pp.  11-32).    This  work 
is  a  standard  one,  but  only  for  the  state  of  knowledge  on  the  subject  when  it 
was  written. 


Fol's  famous  *  Quadrille  oi  Centres/  which  the  two  halves  of 
the  male  and  female  centrosomes  were  supposed  to  dance  round 
the  segmentation  nucleus  of  the  fertilised  egg-cell,  has  proved 
to  be  erroneous^  Strasburger  and  his  followers l  "think  that 
centrosomes  are  wanting  in  the  higher  kinds  of  plants, 
and  in  the  division  of  Protozoa  they  are  either  altogether 
absent  or  of  rare  occurrence.  They  are  present  in  the 
segmentations  of  the  nucleus  which  lead  to  the  formation 
of  spindle-poles  before  fertilisation  in  the  sun-animalculae 
(Actinosphaerium)  .3 

If  centrosomes  were  absolutely  essential  to  the  action  of 
heredity,  they  would  inevitably  be  present  whenever  cells 
divide,  or  at  least  whenever  those  cells  divide  which  are  con- 
nected with  the  preservation  of  the  species,  and  this  is  not 
the  case. 

The  whole  question  of  the  function  of  centrosomes  is  still 
involved  in  much  obscurity,  and  Strasburger  sums  up  the^ 
difficulties  admirably  in  the  following  words  :  3  '  At  the  present 
moment  and  at  the  present  state  of  our  investigations,  I  must 
content  myself  with  the  thought  that  individualised  centro- 
somes disappear  in  the  more  highly  organised  plants.  Why 
otherwise  should  we  fail  to  trace  them  in  any  of  the  Pterido- 
phyta  and  Phanerogams,  whilst  we  succeed  in  the  Bryophyta, 
(Mosses)  ?  I  am  quite  willing  to  agree  with  Flemming,  who 
thinks  it  possible  that  in  the  future  centrosomes  will  be  found 
also  in  the  higher  plants.  ...  No  one  as  yet  has  been  able  to 
form  a  conclusive  opinion  regarding  the  origin,  structure, 
function,  persistence  or  disappearance  of  the  centrosomes 
whilst  the  cell  is  at  rest,  nor  is  much  known  as  to  their  dis- 
tribution, although  the  reasons  brought  forward  by  Flemming 
for  believing  them  to  occur  everywhere  seem  very  weighty, 
when  considered  separately.  Carnoy,  however,  takes  a  decidedly 
opposite  view.'v 

We  must  refer  our  readers  to  Wilson  and  0.  Hertwig  for 
further  information  on  the  subject  of  centrosomes.  These 
two  writers  have  collected  a  quantity  of  material  involving 

1  Histologische  Studien  aus  dem  Banner  Botanischen  Institut,  Berlin,  1897. 

2  0.  Hertwig,  Allgemeine  Biologie,  1906,  p.  189.    * 

3  '  tiber    Reduktionsteilung,    Spindelbildung,    Centrosomen    und    Cilien- 
bildner  im  Pflanzenreich '  (Histolog.  Beitrdge,  1900,  Part  6,  pp.  170,  171). 

H  2 


much  research.  Strasburger  concludes  with  a  reference  to  a 
theory  based  on  recent  research,  according  to  which  the  cen- 
trosome  is  a  mass  of  kinoplasm,  not  only  serving  the  purpose 
of  cell-division,  but  also  concerned  in  the  movement  of  the 
flagella  and  cilia  of  many  cells  and  especially  of  the  spermatozoa. 
0.  Hertwig  has  adopted  this  view  in  his  '  Allgemeine  Biologie,' 
1906,  p.  122,  &C.1 

As  Strasburger  says  in  the  above  quotation,  we  still  know 
very  little  as  to  the  origin  of  the  centrosomes.  Some  regard 
them  as  composed  of  the  protoplasm  of  the  cell ;  others,  with 
more  probability,  think  that  they  are  a  product  of  the  nucleus. 
A  new  theory  is  that  the  centrosomes  are  not  permanent  con- 
stituents of  the  cell,2  but  are  merely  microsomes,  representing 
a  part  of  the  achromatic  framework  of  the  cell  or  nucleus, 
which  have  a  temporary  importance  during  the  processes 
involved  in  karyokinesis,  inasmuch  as  such  a  microsome,  by 
taking  up  its  position  at  the  pole  of  the  nucleus  in  course  of 
division,  becomes  the  focus  of  the  protoplasmic  rays  from 
which  the  spindle  proceeds.  If  this  theory  is  true,  the  cen- 
trosomes, and  the  attraction  sphere  which  they  form,  are 
perhaps  not  the  causes  of  nuclear  division,  but  a  result  of  the 
beginning  of  the  process.  Mitrophanow  tried  to  prove  this 
theory  as  early  as  1894,  in  his  *  Contribution  a  la  division 
cellulaire  indirecte  chez  les  Selaciens  '  (Journal  international 
d'anatomie  et  de  physiologic,  XI). 

Wasilieff  thinks  that  the  centrosome  is  only  a  temporary 
product  of  the  joint  action  of  nucleus  and  protoplasm  ;  3  and 
this  theory  is  supported  by  experiments  (to  which  reference 
will  be  made  in  the  next  chapter)  by  Morgan,  Loeb  and 
Wilson,  who  succeeded  in  artificially  producing  centrosomes 
in  the  unfertilised  eggs  of  sea-urchins  by  means  of  salt 

The  astral  rays  of  the  nuclear  spindle  may  all  be  formed  of 

1  See  also  Ikeno,  '  Bleptutoplasten  im  Pflanzenreich '  (Biolog.  Zentralblatt, 
XXIV,  1904,  No.  6,  pp.  211-221).     Recent  investigations  made  by  Russo  and 
di  Mauro  in  1905,  and  by  Gemelli  in  1906,  seem  however  to  show  that  the 
flagella  and  cilia  are  not  connected  with  the  centrosomes,  but  with  special 
basal  bodies  formed  by  a  thickening  of  the  cell- wall. 

2  Cf .  the  views  expressed  by  Brandes  and  Flemming  in  the  Verhandlungen  der 
Deutschen  Zoolog.  Gesellsckafi,  1897,  pp.  157-162. 

3  '  tiber  kiinsthche   Parthenogenesis  des   Seeigeleis  '  (Biolog.    Zentralblatt, 
XXII,  1902,  No.  24,  pp.  758,  &c.). 


the  achromatic  nuclear  framework,  or  of  the  spongioplasm 
of  the  cell-body,  or  they  may  have  a  mixed  origin.1 

We  really  know  nothing  of  the  cause  producing  this  radia- 
tion, nor  do  we  know  what  makes  the  V-shaped  loops  of 
chromatin  split  in  half  lengthwise.3 

The  only  certain  facts  are  that  karyokinesis  depends  upon 
the  partition  of  the  chromosomes,  and  that  the  protoplasmic 
rays  of  the  nuclear  spindle  determine  the  direction  in  which 
the  chromosomes  move.  We  are  also  convinced  that  great 
importance  in  the  processes  of  evolution  must  be  assigned  to 
the  persistence  in  the  number  of  chromosomes  contained  in 
the  somatic  cells  of  individuals  belonging  to  one  and  the  same 
species,  which  number  is  most  accurately  preserved  during 
karyokinesis  by  the  longitudinal  division  of  the  chromatin 
loops.  If  we  compare  this  normal  form  of  mitosis  with  the 
method  of  dividing  the  chromatin  in  the  germ-cells  (cf .  the  next 
chapter)  we  shall  lay  still  greater  stress  upon  the  importance 
of  this  point.  We  must,  however,  remember  that  the  science 
of  the  present  day  is  quite  unable  to  tell  us  anything  about 
the  inner  causes  that  produce  the  wonderfully  complicated 
phenomena  observed  in  indirect  karyokinesis. 

'  We  must  acknowledge  that  we  are  not  in  a  position  to 
form  any  plausible  theory  at  all  as  to  the  kind  of  reciprocal 

1  Cf .  Henking,  '  tlber  plasmatische  Strahlungen  '  ( Verhandl.  der  Deutschen 
Zoolog.    Gesellschaft,  1891,  pp.   29-36) ;    also  Yves  Delage,  La  structure  du 
protoplasma,  1895,  p.  75  ;    O.  Hertwig,  Allgemeine  Biologie,  pp.  192,  etc. 

2  Cf.  also  H.  E.  Ziegler,  '  Untersuchungen  iiber  die  Zellteilung  '  ( Verhandl. 
der  Deutschen  Zoolog.  'Gesellschaft,    1895,   pp.    62-83.)     A  great  number  of 
theories  have  been  advanced  to  account  for  the  nuclear  figures  in  karyokinesis, 
but  none  of  them  can  claim  a  high  degree  of  probability.     This  remark  applies 
to  Ziegler's  own  comparison  of  these  figures  with  the  lines  of  force  in  a  magnetic 
field.     Yves  Delage  (pp.  310-314)  gives  a  good  summary  and  criticism  of  the 
various  theories  regarding  the  causes  of  cell-division  and  of  the  formation  of 
karyokinetic  figures.     He  says  with  much  truth  of  the  comparatively  best 
of  these  theories — that,  viz.,  advanced  by  Henking — that  it   would   be  just 
as  reasonable  to  see  in  the  lion,  the  scales,  and  the  fish  of  the  zodiac  a  real 
lion,  real  scales  and  real  fish,  as  to  act  like  the  propounders  of  these  theories, 
and   pretend   that   their   mechanical   representations    of   cell-structures   and 
karyokinetic  figures  are  real  cell-structures  and  real  figures.     Another  attempt, 
no  more  satisfactory  than  its  predecessors,  at  explaining  the  mechanism  of 
cell-division  has  been  made  quite  recently  by  V.  Schlapfer  in  his  article  'Eine 
physikalische  Erklarung  der  achromatischen  Spindelfigur  und  der  Wanderung 
der  Chromatinschleifen  bei  der  indirekten  Zellteilung  '  (Archiv  fur  Entwick- 
lungsmechanik,  XIX,  1905,  pp.  107-128).     It  is  an  undoubted  fact  that  many 
physical  and  chemical  influences  are  at  work  in  the  process  of  karyokinesis, 
but  we  possess  as  yet  very  little  real  knowledge  of  their  power  to  direct  and 
further  the  biological  aim  of  the  division  of  cell  and  nucleus. 


action  existing  between  the  cell-body  and  the  nucleus.  We 
have  no  foundations  of  facts  upon  which  to  construct  a  theory.' l 

Whoever  cares  to  see  a  summary  and  criticism  of  the  various 
hypotheses  regarding  the  mechanism  of  mitosis  propounded  by 
E.  van  Beneden,  Heidenhain,  B.  Hertwig,  Fol,  &c.,  may  refer 
to  Wilson,  '  The  Cell,'  pp.  100-111.  His  resume  of  the  whole 
discussion  is  as  follows  :  '  A  review  of  the  foregoing  facts 
and  theories  shows  how  far  we  still  are  from  any  real  under- 
standing of  the  process  involved  either  in  the  origin  or  in  the 
mode  of  action  of  the  mitotic  figure  '  (p.  111).3 

The  secret  physiological  causes  that  motive  cell-division  are 
unknown  to  the  scientist,  whose  microscope  reveals  to  him 
only  their  morphological  action.  They  are  a  problem  of 
cellular  physiology,  a  problem  containing  in  itself  the  whole 
mystery  of  life.  We  have  now  to  trace  this  mystery  in  the 
phenomena  of  fertilisation  and  heredity,  and  we  shall  be  able 
to  approach  its  solution  in  Chapter  VIII,  where  we  shall  deal 
with  the  processes  of  organic  development. 

1  Korschelt    and     Heider,     Lehrbuch     der    vergleichenden    Entwicklungs- 
geschichte  (Allgem.  Teil,  Part  I,  pp.  153,  154). 

2  See  also  Wilson's  chapter  on  '  Some  problems  of  cell-organisation.' 



(See  Plates  I  and  II) 




Their  general  features.  Reduction  in  the  number  of  chromosomes  (p.  110). 
Varieties  of  maturation -divisions.  Equal  division  and  reducing 
division.  The  eumitotic  type  (p.  111).  The  pseudomitotic  type  and 
its  subdivisions  (p.  111).  Difficulties  in  interpreting  microscopical 
observations.  Diagrams  representing  the  maturation-divisions  of  the 
egg-cell  (p.  118). 


Echinus  type  and  Ascaris  type  of  nuclear  union  (p.  120).  More  detailed 
description  of  the  process  of  fertilisation  (Boveri)  (p.  123).  Equal 
distribution  of  the  chromatin  nuclear  constituents  of  both  parents 
to  the  segmentation-cells.  Apparent  exceptions  (p.  125).  Boveri's 
view  of  the  importance  of  the  male  centrosome  in  fertilisation 
(p.  126). 



Pathological  and  physiological  polyspermy.  Double-fertilisation  in  the 
Angiosperms  (p.  128).  Specific  polyembryony  (p.  129). 



Conjugation  of  ciliate  Infusoria.  Transition  from  the  conjugation  of 
lower  organisms  to  the  fertilisation  of  higher  organisms  (p.  131). 
Comparative  deductions  (p.  134). 


Variations  in  the  behaviour  of  the  polar  bodies  and  in  the  chromatin 
reduction  (p.  136).  Parthenogenesis  in  the  vegetable  kingdom. 
Conclusipns  (p.  138). 


Account  of  various  experiments  and  their  results  (p.  139).  Behaviour  of 
the  astrospheres  (p.  142).  Bearing  of  these  experiments  upon  the 
problem  of  fertilisation  (p.  144).  Morphological  and  chemico- 
physical  theories  of  fertilisation  (p.  145). 


Account  of  various  experiments  and  their  results  (p.  149).  Boveri's 
'organisms  without  maternal  qualities'  (p.  152).  Ziegler's  experi- 
ments on  the  constriction  of  sea-urchins'  eggs  (p.  153).  Importance 
of  the  spermato-centrosome  in  division  of  the  egg-cell  (p.  154). 




The  essence  of  normal  fertilisation  is  the  union  of  the  egg-  and  sperm  - 
cells  (p.  156).  Normal  fertilisation  compared  with  abnormal  and  with 
parthenogenesis  (p.  157).  Is  the  essential  part  of  the  new  organism 
contained  in  the  egg-cell  alone  or  in  the  sperm-cell  alone,  or  in 
both  ?  (p.  158).  Why  must  the  nuclei  of  two  germ-cells  unite  to 
effect  fertilisation?  Twofold  purpose  of  fertilisation  (p.  160). 
First,  to  stimulate  the  production  of  a  new  individual.  Various 
theories  regarding  rejuvenescence  of  the  organic  substance  through 
the  process  of  fertilisation  (p.  161).  Second  purpose  of  fertilisation, 
to  transmit  to  the  offspring  the  combined  properties  of  both  parents 
(p.  163).  Final  significance  of  the  process  of  reduction  (p.  164). 
Final  significance  of  the  distribution  of  chromatin  at  the  union  of  the 
germ-nuclei  (p.  165).  The  nuclear  chromosomes  the  chief  material 
bearers  of  heredity.  Boveri's  theory  of  the  '  Individuality  '  of 
chromosomes  (p.  167).  Its  connexion  with  Mendel's  Law  (p.  170). 
Object  of  the  combination  of  qualities  effected  by  the  chromosomes 
in  the  process  of  fertilisation  (p.  173).  Criticism  of  Weismann's 
views  regarding  amphimixis  (p.  174).  The  chromosomes  probably 
are  the  bearers  of  the  interior  laws  of  development  governing  organic 
life  (p.  177). 


EVER  since  the  time  of  Aristotle  the  minds  of  men  have 
busied  themselves  with  the  problem  of  fertilisation,  and  with 
the  way  in  which  the  characteristics  of  the  parents  are  handed 
down  from  generation  to  generation  of  their  descendants.  In 
the  last  few  centuries  the  ovulists  and  the  animalculists  have 
argued  with  one  another  as  to  whether  the  ovum  or  the  sperm- 
cell  was  alone,  or  at  least  chiefly,  responsible  for  the  phenomena 
of  fertilisation  and  heredity  ;  the  matter  was  discussed  with 
much  energy  and  varying  success,  and  was  finally  left  un- 
decided, for  neither  party  possessed  the  actual  knowledge 
necessary  to  enable  them  to  arrive  at  a  decision — it  was  reserved 
for  modern  microscopical  research,  with  its  extremely  delicate 
and  ingenious  methods  of  investigation,  to  supply  a  more  or  less 
adequate  basis  for  the  solution  of  these  problems.  Let  us  now 
consider  the  results  of  the  most  recent  research,  and  see  to 
what  conclusions  they  lead.  It  is  interesting  to  observe 
that  many  of  the  newer  theories  of  fertilisation  approximate 
very  closely  to  Aristotle's  opinion,  which  was  that  the  female 
element  supplied  the  material  out  of  which  the  new  individual 
was  formed,  whilst  the  male  element  supplied  the  impulse 
to  its  development.  This  coincidence  of  ideas  must  not, 
however,  in  any  way  influence  us  in  judging  these  theories 


During  the  last  few  years  more  new  facts  have  been  ob- 
served, more  experiments  made,  more  theories  invented  and 
published  on  the  problems  of  fertilisation  and  its  relation  to 
heredity,  than  perhaps  on  any  other  subject  of  scientific 
research.1  We  need  not  trouble  about  the  purely  speculative 
theories,  but  discuss  only  the  scientific  material  from  which  the 
supports  for  the  theoretical  superstructure  are  taken.  We 
shall  consider  the  nature  of  these  supports,  and  see  how  far 
anyone  has  yet  succeeded  in  uniting  them  so  as  to  give  us  any 
conception  of  the  structure,  which  it  will  be  the  task  of  future 
generations  to  complete.  But  here  at  once  we  find  ourselves 
involved  in  difficulties.  Who  is  a  trustworthy  guide  in  this 
investigation  ?  Who  can  give  us  information  regarding  the 
quality  of  the  building  materials  and  the  best  mode  of  com- 
bining them,  so  as  to  form  at  least  the  foundation  of  the  future 
edifice  ?  If  we  take  one  of  the  industrious  workmen  as  our 
guide,  there  is  some  danger  lest  he  show  us  especially  the  stones 
that  he  himself  has  hewn  and  fashioned,  and  give  us  a  partial 
account  of  the  reasons  why  these  stones  must  be  used  in  one 
way,  and  not  in  another.  If,  on  the  other  hand,  we  take  a 
number  of  the  workers  as  guides,  their  explanations  may 
involve  contradictions  which  we  cannot  solve.  If  we  have 
recourse  to  one  of  the  theorising  inspectors,  we  inevitably 
expose  ourselves  to  the  risk  of  falling  too  much  under  his 
influence  and  accepting  his  interpretations,  to  the  neglect  of 
other,  no  less  well  grounded,  opinions.  Where  are  we  to  find 
an  *  impartial  expert '  on  the  subject  ? 

Of  all  the  recent  publications  in  this  department  of  research 
none  perhaps  is  better  calculated  to  give  a  fair  objective 
account  of  it  than  the  '  Allgemeiner  Teil '  (General  Section)  of 
Korschelt  and  Heider's  '  Vergleichende  Entwicklungsgeschichte 
der  wirbellosen  Tiere  '  (*  Text-book  of  the  Embryology  of 
Invertebrates  ').2  The  authors  have  not  only  shown  marvellous 
industry  in  collecting  and  tabulating  an  immense  number  of 
facts,  but  they  have  also  displayed  great  circumspection  in 
their  critical  appreciation  of  the  various  attempts  to  explain 
these  facts  theoretically. 

1  A  list  of  works  on  this  subject  is  given  by  Y.  Delage,  Korschelt  und  Heider, 
and  E.  B.  Wilson. 

2  Part  I,  Jena,  1902  ;  Part  II,  Jena,  1903.     The  '  General  Section  '  has  not 
been  translated  into  English. 


We  have  frequently  referred  also  to  Y.  Delage's  '  La  structure 
du  protoplasma  et  les  theories  sur  1'heredite  et  les  grands  pro- 
blemes de  la  biologie  generate '  (Paris,  1895).  It  is  of  great 
importance  as  enabling  us  to  follow  the  questions  propounded, 
although  I  cannot  without  reserve  accept  the  author's  own 
'  theorie  des  causes  actuelles.'  1 

E.  B.  Wilson's  book,  '  The  Cell  in  Development  and  Inherit- 
ance '  (New  York,  1902),  contains  a  very  good  resume  of  the 
phenomena  of  fertilisation  and  their  connexion  with  inherit- 
ance ;  and  on  this  subject  I  can  cordially  recommend  Oskar 
Hertwig's  '  Allgemeine  Biologie,'  Jena,  1906,  chapters  11-13. 
Much  has  been  done  by  E.  Strasburger3  and  J.  Eeinke3  to 
facilitate  a  comparison  of  the  results  obtained  by  zoologists 
with  the  analogous  phenomena  observed  by  botanists. 

I  propose  to  discuss  the  points  of  the  subject  in  the  following 
order  :— 

1 .  What  are  the  problems  to  be  solved  ? 

2.  How    do    the    maturation-divisions    of    the    germ-cells 

differ  from  the  ordinary  processes  of  indirect  division 
of  the  nucleus  ? 

3.  What  is  the  normal  process  of  fertilisation  in  an  animal 

egg,  as  a  result  of  the  union  of  the  egg-cell  and  sperm- 
cell  ? 

4.  In  what  relation  do  the  phenomena  of  superfetation  in 

1  A  later  edition  of  the  same  work  was  published  in  Paris,  1903,  entitled  : 
UHeredite  et  les  grands  problemes  de  la  biologie  generale.      A  review  of  the 
theories  of  fertilisation,  mixed  with  a  good  deal  of  the  hypothetical  element, 
was  given  by  Delage  in  his  address  '  Les  theories  de  la  fecondation,'  delivered 
at  the  Fifth  International  Zoological  Congress  in  Berlin  (August  1901)  and 
printed  in  the  Verhandlungen  of  the  same  Congress  at  Jena,  1902  (pp.  121-140). 
Cf.  also  a  lecture  delivered  by  Delage  in  Paris  on  April  10,  1905,  on  '  Les 
problemes  de  la  biologie  '  (Bull,  de  Vlnstit.  general  psychologique,  V,  1905,  No.  3, 
pp.  215-236).    In  an  oration  at  the  seventy- third  meeting  of  German  naturalists 
and  physicians  in  September  1901,  entitled  'Das  Problem  der  Befruchtung  ' 
(Jena,  1902),  Boveri  expounded  chiefly  his  own  views  on  the  subject.     At 
the  thirteenth  annual  meeting  of  the  German  Zoological  Society  in  June  1903, 
he  read  a  paper  on  the  constitution  of  the  chromatin  nuclear  substance  ('  Uber 
die  Konstitution  der  chromatischen  Kernsubstanz,'   Verhandl.  pp.  10-33),  in 
which  he  developed  his  views  regarding  the  individuality  of  the  chromosomes. 
In  the  course  of  this  chapter  we  shall  have  occasion  to  refer  to  the  works  of 
several  other  scientists.     L.   Katheriner  contributed  a  good  review  of  the 
attempts  to  solve  the  problem  of  heredity  to  Natur  und  Offenbarung,  1903, 
pp.  513,  &c. 

2  *  Histologische  Beitrage,'  No.  6  :      Uber  Reduktionsteilung,  Spindelbildung, 
Centrosomen  und  Cilienbildner  im  Pflanzenreich,  Jena,  1900. 

3  Einleitung   in  die   theoretische  Biologie,  chapter   34,   '  Morphologic   der 


animals  stand  to  those  of  double  fructification  in 
plants  ? 

5.  What  are  the  points  of  resemblance  between  the  ferti- 

lising processes  of  multicellular  animals  and  plants 
and  the  phenomena  of  conjugation  observed  in  uni- 
cellular organisms  ? 

6.  What  light  is  thrown  on  the  problem  of  fertilisation 

by  the  facts  of  natural  parthenogenesis  ? 

7.  Experiments  in  artificial  parthenogenesis. 

8.  Attempts  to  fertilise  non-nucleated  fragments  of  eggs. 

9.  What  conclusions  may  be  deduced  from  this  series  of 

phenomena  with  regard  to  fertilisation  in  general,  and 
our  knowledge  of  the  material  bearers  of  heredity  ? 


What  is  it  that  enables  living  organisms  to  propagate 
their  species  ?  The  power  of  propagation  depends  upon  the 
possession  of  germ-plasm,  which  is  the  means  of  preservation 
of  species.  In  unicellular  organisms  the  germ-plasm  is  contained 
in  the  cell  that  constitutes  the  body  ;  but  in  multicellular 
animals  and  plants  there  are  distinct  germ-cells,  out  of  which 
the  body  of  the  new  individual  is  formed.  The  plasm  of 
these  cells,  called  by  Nageli  idioplasm  and  by  Weismann 
germ-plasm,  is  therefore  the  actual  bearer  of  the  phenomena 
of  heredity.  Weismann  has  based  upon  this  fact  his  well- 
known  theory  of  the  continuity  of  germ-plasm.1  He  believes 
that  within  the  tiny  mass  of  organic  substance  in  the  germ-cell, 
and  especially  within  its  nucleus,  are  contained  the  material 
constituents  for  the  formation  of  new  individuals,  and  that 
these  constituents  are  transmitted  from  generation  to  genera- 
tion. He  calls  these  constituents  idants,  ids,  determinants 
and  biophors,  according  to  their  size ;  biophors  regularly 
arranged  compose  determinants,  these  form  ids  (which  contain 
all  the  primary  constituents  necessary  to  the  production 
of  an  individual),  and  the  ids  finally  combine  to  make  up 
idants.  This  speculation  of  Weismann's,  according  to  which 
germ-plasm  is  in  some  degree  an  extremely  delicate,  artificial 

1  Weismann  has  given  a  detailed  account  of  his  theory  in  his  lectures  on 
the  evolution  theory,  17th  lecture  (Vol.  I,  pp.  345,  &c.,  Eng.  trans.). 


sort  of  mosaic,  is  the  foundation  of  his  Preformation  theory.1 
Opposed  to  this  theory  are  the  epigenetic  views  of  0.  Hertwig, 
Y.  Delage,  Hans  Driesch  and  others,3  who  believe  the  develop- 
ment of  the  embryo  to  be  determined,  not  by  material  deter- 
mining constituents,  but  by  dynamic  causes,  such  as  definite 
chemical  and  physical  properties  of  the  germ-plasm.3 

J.  Eeinke  has  combined  with  this  theory  that  of  Dominants, 
which,  after  the  fashion  of  teleological  entelechies,  direct  and 
control  the  activity  of  the  mechanical  energies.4  Driesch 
inclines  to  a  similar  opinion,  as  he  upholds  the  autonomy 
of  the  vital  processes,  and  thinks  they  cannot  be  accounted 
for  by  mechanical  causes.5  All  these  theories,  which  I  cannot 
now  discuss  in  greater  detail,  have  been  advanced  as  supplying 
answers  to  one  and  the  same  question  :  '  How  can  we  explain 
the  morphological  processes,  which  present  themselves  to  our 
consideration,  when  we  observe  the  phenomena  of  fertilisation 
and  heredity  in  the  germ-plasm  ?  ' 

A  second  very  interesting  question  is  :  'In  the  case  of  the 
higher  animals  and  plants,  which  require  the  action  of  both 
sexes  for  their  propagation,  why  is  the  ovum  or  the  sperm-cell 
alone  insufficient  for  embryonic  development  ?  Why  is  fertilisa- 
tion necessary  to  the  development  of  the  ovum  ?  Is  the  union 
of  the  two  germ-cells,  which  takes  place  at  fertilisation,  essential 
to  the  beginning  of  embryonic  development,  or  is  the  object  of 
it  to  secure,  by  means  of  bisexual  propagation  (which  Weismann 
calls  amphimixis),  the  advantages  of  a  twofold  inheritance,  and 
a  mixture  of  the  qualities  of  both  parents  ?  Finally,  what 
are  the  real  bearers  of  heredity  in  the  germ-cells  ?  May  we 

1  Preformation,    because,    according  to  it,  every  part  of  the  future  in- 
dividual is  formed  beforehand,  or  rather  determined  beforehand,  by  means 
of  most  minute  determining  constituents  in  the  germ-cell. 

2  Epigenesis  =  development  through  new  formations  ;   according  to  these 
theories  the  various  processes  of  development  in  the  embryo  depend  upon 
new  formations,  produced  by  the  joint  action  of  external  stimuli  and  internal 
dynamic  factors. 

3  The  problem  of  determination,  i.e.  the  question  whether  preformation 
or  epigenesis  lies  at  the  root  of  organic  development,  is  obviously  not  limited 
to  the  beginning  of  the  development  of  the  germ,  but  covers  the  whole  course 
of  ontogeny  (individual  development).     Cf.  Korschelt  and  Heider,  Lehrbuch 
der    vergleichenden     E ntwicklungsgeschichte    der    wirbellosen    Tierc,    Part    I, 
pp.  81-160.     The  problem  of  determination  will  be  dealt  with  more  fully  in 
Chapter  VIII,  *  The  Problem  of  Life.' 

4  Reinke,  Die  Welt  als  Tat,  Berlin,  1903,  pp.  275-292 ;  also  '  Die  Dominanten- 
Jehre,'  in  Natur  und  Schule,  1903,  Parts  6  and  7. 

5  Driesch,  Die  organischen  Regulationen,  Leipzig,  1901. 


regard  the  chromosomes  of  the  nucleus   as  such,  and  with 
what  justification  ?  ' 

We  will  now  try  to  examine  these  questions  more  closely 
from  the  standpoint  of  the  morphological  processes  in  the 
germ-cells,  as  revealed  by  the  microscope.  Even  if  we  fail  to 
arrive  at  any  final  explanation,  it  is  nevertheless  important 
to  see  how  far  scientific  research  on  this  subject  has  advanced. 
We  must  begin  with  the  phenomena  of  maturation  in  the 


Both  the  ovum  and  the  spermatozoon  must,  before 
becoming  capable  of  fertilisation,  undergo  two  divisions,  which 
are  known  as  maturation-divisions.  Let  us  consider  first  those 
of  the  ovum. 

As  Y.  Delage  rightly  remarks,  what  we  generally  call  a 
mature  egg,  is  really  the  grandmother  of  the  egg-cell.  At 
that  stage  the  egg  is  termed  a  primary  oocyte  ;  after  the  first 
maturation-division  it  becomes  a  secondary  oocyte,  and  after 
the  second  division  it  is  an  egg  capable  of  fertilisation.  This 
process  of  twofold  division  differs  entirely  in  many  respects 
from  the  usual  form  of  division  of  cell  and  nucleus,  as  described 
in  the  preceding  chapter.  As  a  rule,  the  division  of  a  mother- 
cell  produces  two  daughter-cells  of  equal  size,  and,  when  they 
subdivide,  four  granddaughter-cells,  all  of  the  same  size,  are 
formed  ;  but  the  two  maturation-divisions  of  the  egg-cell 
result  in  the  formation  of  one  large  cell,  which  is  the  ovum 
proper,  and  of  two,  or  strictly  speaking  three,1  diminutive  cells 
or  portions  of  cells,  called  polar  bodies.  In  the  ordinary 
course  of  indirect  cell-division  a  period  of  rest  intervenes 
between  two  divisions,  during  which  period  the  nucleus 
resumes  its  normal  shape  ;  but  there  is  no  resting  stage  between 
the  two  maturation-divisions  ;  the  second  generally  takes 
place  immediately  after  the  first,  and  for  this  reason  the 
separation  of  the  polar  bodies  from  the  ovum  has  been  termed 
'  precipitate  cell-division.'  Finally,  in  the  normal  form  of 

1  The  first  polar  body  often  divides  again  immediately  after  its  separation 
from  the  ovum,  so  that,  when  the  second  polar  body  is  formed,  there  are  in 
all  three  minute  bodies  present  besides  the  ovum. 


karyokinesis,  the  original  number  of  chromosomes  persists  in 
the  daughter-cells ;  in  maturation-division  of  the  germ-cell, 
it  is  a  remarkable  fact,  that,  after  the  separation  of  the  polar 
bodies,  the  nucleus  of  the  mature  germ-cell  contains  only  half 
the  number  of  chromosomes  that  occur  in  the  somatic  cells 
of  the  same  individual,  and  at  the  same  time  the  amount  of 
chromatin  originally  in  the  nucleus  is  generally  reduced  to  a 
quarter.  This  reduction,  but  more  particularly  that  in  the 
number  of  chromosomes,  leads  us  to  speak  of  the  processes  of 
reduction,  which,  as  will  be  seen  later,  appear  to  be  of  very 
great  significance  in  the  problem  of  fertilisation. 

Like  the  egg-cell,  the  sperm-cell  undergoes  a  twofold 
division  in  the  course  of  maturation.  The  primary  spermato- 
cyte  by  indirect  karyokinesis  gives  rise  to  two  secondary 
spermatocytes,  and  each  of  these  divides  into  two  spermatids 
or  ripe  sperm-cells,  so  that  in  this  case,  too,  the  primary  sper- 
matocyte  has  four  descendants.  But  whereas  the  four  descend- 
ants of  the  primary  oocyte  are  of  unequal  size  and  value,  and 
only  one,  the  ripe  ovum  itself,  is  concerned  with  fertilisation, 
those  of  the  primary  spermatocyte  are,  as  a  rule,  all  four  of 
equal  size,  each  able  to  fertilise  an  ovum.1 

It  is  a  most  important  fact  that,  at  the  completion  of  the 
processes  of  maturation,  the  number  of  chromosomes  in  both 
sperm  and  egg-cells  is  reduced,  so  that  the  mature  cell  contains 
only  half  the  number  that  are  present  in  the  somatic  cells  of 
the  same  individual  and  of  the  same  species.  The  bearing  of 
this  fact  upon  fertilisation  will  be  shown  later.3 

1  I  say  '  as  a  rule,'  because  Meves  believes  that  he  has  recently  observed 
a  formation  of  polar  bodies  during  the  maturation-divisions  of  sperm-cells. 
Cf.  F.  Meves,  '  Richtungskorper  in  der  Spermatogenese  '  (Mitteil.  d.  Vereins 
Schleswig-Holsteiner  Arzte,  XI,  1903,  No.  6) ;    '  Uber  Richtungskorperbildung 
im  Hoden  von  Hymenopteren '  (Anatom.  Anzeiger,  XXIV,  1903,  pp.  29,  &c.). 

2  I  may  incidentally  remark  that  during  the  maturation-divisions  of  the 
sperm-cells  of  many  animals,  and  especially  of  many  insects,  the  presence  of 
accessory  or  heterotropic  chromosomes  has  been  observed,  the  use  of  which 
has  not  hitherto  been  satisfactorily  explained.     See  Korschelt  und  Heider, 
Lekrbuch   der   vergl.   Entwicklungsgeschichte,    &c.,   601.     R.    de   Sinety,    S.J., 
has  traced  the  history  of  these  accessory  chromosomes  very  carefully  in  his 
Recherches  sur  la  biologic  et  Vanatomie  des  Phasmes,  Lierre,  1901  ;  and  so  has 
Sutton,  an  American  scientist,  in  his  study  of  a  grasshopper  (Brachystola 
magna).     Montgomery  gives  the  accessory  chromosomes,  discovered  by  him 
in  Hemiptera,  the  name  of  heterochromosomes.     See  also  Stevens,  '  Studies  in 
Spermatogenesis,  with  especial  reference  to  the  accessory  chromosome '  (Carnegie 
Institution,    Washington,    September    1905).      E.    B.    Wilson    has    recently 
published  some  important  articles  on  the  various  forms   of  chromosomes 
occurring  in  Hemiptera,  dividing  them  into  idiochromosomes  (of  which  there 


Very  various  opinions  exist  as  to  the  time  and  manner  in 
which  the  reduction  in  the  number  of  chromosomes  takes  place  ; 
this  may  partly  be  accounted  for  by  the  fact  that  different 
scientists  have  chosen  different  objects  for  observation.  We 
must  content  ourselves  with  a  condensed  summary  of  the 
facts,  based  chiefly  upon  Korschelt  and  Heider  (pp.  572, 


We  must,  in  theory,  distinguish  two  forms  of  maturation- 
division  of  germ-cells,  viz.  those  called  by  Weismann  '  equation  ' 
or  equal  division,  and  reducing  division.  The  former  follows 
the  ordinary  laws  of  karyokinesis,  in  which  each  chromosome 
of  the  mother-nucleus  splits  lengthwise,  thus  enabling  each 
daughter-nucleus  to  have  the  same  number  of  chromosomes  as 
there  were  in  the  mother-nucleus,  whence  this  kind  of  division 
is  called  equal.  Eeducing  division  is  altogether  different. 
When  it  takes  place,  whole  chromosomes  are  distributed  to 
the  daughter-nuclei,  so  that  there  is  a  reduction  in  the  original 
number  of  chromosomes,  each  daughter-nucleus  having  only 
half  as  many  as  the  mother-nucleus. 

When  the  two- successive  divisions  of  the  germ-cell  are  both 
equal,  the  whole  maturation-division  is  called  eumitotic,  because 
it  follows  the  normal  type  of  mitosis.3  If,  on  the  other  hand, 
at  least  one  of  the  two  divisions  is  a  reducing  division,  the 
whole  process  of  maturation-division  is  called  by  Korschelt 
and  Heider  pseudomitotic,  and  we  may  accept  this  name. 
Three  varieties  of  pseudomitotic  division  must  be  dis- 
tinguished. The  reducing  division  may  follow  the  equal 
division,  and  then  we  have  a  case  of  post-reduction  division  ; 
or  the  reducing  division  may  precede  the  equal  division, 
and  then  we  have  a  case  of  pre-reduction  division  ;  or  both 

are  various  sizes)  and  heterotropic  chromosomes,  and  discussing  their  biological 
functions.  ('  Studies  on  Chromosomes,'  in  the  Journal  of  Experimental 
Zoology,  IT,  Nos.  3  and  4,  III,  No.  1).  In  the  last  section  of  this  chapter  we 
shall  refer  again  to  the  accessory  chromosomes. 

1  In  one  of  his  recent  works,   '  Uber  die  Konstitution  der  chromatischen 
Kernsubstanz,'  in  the  Verhandl.  der  Deutschen  Zoolog,   Gesellschafl  for  1903, 
Boveri  describes  the  statement  of  the  reduction  problem  given    by  these 
two  authors  as  a  'model.'     Cf.  also  O.  Hertwig,  Allgemeine  Biologie.  1906, 
pp.  282,  etc. 

2  I  cannot  here  discuss  the  varieties  of  eumitotic  division  known  as  homceo- 
typic  and  heterotypic.     In  the  former  a  real  separation  of  the  two  halves  of 
the  split  chromosome  takes  place,  in  the  latter  they  remain  connected  by  their 
ends,  so  that  the  two  half-loops  form  a  ring.     Such  chromosomes  are  termed 
'  heterotypic.' 


divisions  may  be  reducing,  and  the  process  may  be  called 
one  of  double  reducing,  or  a  bireduction  division.1 

These  various  kinds  of  maturation-division  have  a  direct 
bearing  upon  the  problem  when,  and  how,  the  original  number 
of  chromosomes  in  the  somatic  cells  is  reduced  to  half  that 
number  in  the  egg  and  sperm-cells  at  the  conclusion  of  the 
process  of  maturation. 

In  eumitotic  maturation-division,  the  reduction  does  not 
take  place  during  the  divisions,  but  precedes  them.  The 
primary  oocytes  and  spermatocytes  have  in  this  case  the 
reduced  number  of  chromosomes,  before  they  begin  to  divide 
further.  We  know  absolutely  nothing  as  to  the  manner  in 
which  this  reduction  is  effected,  and  very  little  as  to  the  time 
when  it  takes  place.  In  many  plants  and  animals  it  seems 
to  occur  very  early,  during  generations  of  cells  preceding  the 
formation  of  germ-cells.2 

In  pseudomitotic  maturation-division,  the  chromatin  re- 
duction takes  place  automatically  by  means  of  one  or  both 
processes  of  division,  but  the  manner  in  which  it  is  effected  is 
still  very  obscure,  and  various  authors  do  not  agree  in  their 
interpretation  of  their  microscopical  observations. 

The  actual  results  obtained  stand  in  the  following  relation 
to  the  theoretical  kinds  of  maturation-division  that  have  been 
described  above.  The  eumitotic  type — in  which  both  matura- 
tion-divisions are  produced  by  longitudinal  splitting  of  the 
chromosomes,  so  that  no  reduction  in  the  number  of  chromo- 
somes is  caused  actually  by  the  divisions — seems  to  occur  very 
frequently  in  both  animals  and  plants.  Some  authors  are 
inclined  to  think  that  this  type  might  prove  to  be  universal,  if 
we  could  explain,  in  accordance  with  it,  the  microscopical 
observations  that  have  hitherto  been  interpreted  in  the  pseudo- 
mitotic sense. 

Boveri,  whose  brilliant  research  work  on  Ascaris  and  other 
creatures  has  caused  the  eumitotic  maturation-division  to  be 
known  also  as  the  '  Boveri  type  of  division,'  emphatically 

1  I  have  ventured  to  coin  this  word  to  designate  the  double  reducing  division, 
forming  it  on  the  analogy  of  the  other  names  given  to  division. 

2  Cf.  Wilson,  The  CeH,pp.  272,  &c.,  also Strasburger,  Uber Eeduktionsteilung, 
Spindelbildung,  &c.,  Jena,   1900,  pp.  81,  &c.     Strasburger  does  not  call  the 
reduced  number  of  chromosomes  in  the^germ-cells  reduced,  but  original.     This 
may  possibly  be  correct  phylogenetically,   but  it  can  scarcely  be  justified 
ontogenetically,  at  least  in  the  case  of  multicellular  animals. 


maintains  that  the  reduction  in  the  number  of  chromosomes 
does  not  take  place  during  the  maturation-divisions,  nor  is  it 
due  to  them,  but  precedes  them,  inasmuch  as  in  the  primary 
oocytes  and  spermatocytes  the  number  of  chromosomes  is 
always  half  that  of  the  chromosomes  in  the  somatic  cells  of 
the  same  individual.  The  Ascaris  megalocephala  var.  bivalens, 
chosen  by  Boveri  for  investigation,  has  two  chromosomes  in 
each  of  its  primary  germinal  vesicles,  each  consisting  of  four 
grains  of  chromatin,1  which  Boveri  believes  to  have  been 
formed  by  a  double  longitudinal  division  of  the  original  chromo- 
some. This  division  is  prepared  in  the  nucleus  of  the  primary 
germ-cells,  and  is  effected  by  the  two  maturation-divisions,  so 
that  finally  the  mature  ovum  and  spermatozoon  contain  each 
two  chromosomes  in  their  nucleus,  i.e.  the  same  number  as 
before,  whilst  the  somatic  cells  contain  four. 

The  eumitotic  type  of  maturation-division  of  the  germ- 
cells  has  been  described  by  many  zoologists  ;  by  0.  Hertwig 
and  A.  Brauer  (in  Ascaris),  by  Meves,  McGregor,  Janssens, 
Eisen,  Carnoy  and  Lebrun  (in  Amphibia),  Ebner  and  von 
Lenhossek  (in  the  rat),  de  Sinety  (in  Orthoptera),  &c.  Many 
eminent  botanists,  too,  and  especially  Strasburger,  with  whom 
Guignard,  Motier  and  Juel  agree,  concur  in  believing  the 
maturation-divisions  of  plants  to  be  of  the  eumitotic  type,  as 
they  take  place  by  a  twofold  longitudinal  splitting  of  the 
chromosomes,  and  these  writers  are  of  opinion  that  the  re- 
duction in  the  number  of  chromosomes  is  effected  before 
the  maturation-divisions,  viz.  in  the  embryo-sac,  or  at  the 
formation  of  the  pollen. 

Pseudomitotic  maturation- division  has  hitherto  been 
observed  chiefly  in  Arthropods. 

Post-reduction  division,  in  which  the  first  of  the  two 
maturation-divisions  is  equal,  and  the  second  reducing,  is 

1  It  would  perhaps  be  well  for  this  reason  to  adopt  the  number  8  for  the 
chromosomes  of  the  nucleus  of  the  primary  germ-cell,  as  Kathariner  has  done 
in  his  article  in  Natur  und  Offenbarung,  1903,  pp.  524,  527.  The  adoption  of 
this  number  would,  however,  lead  to  the  following  difficulties.  First,  in 
Ascaris  megalocephala  var.  bivalens,  the  primary  germ  cells  would  contain 
twice  as  many  chromosomes  as  the  somatic  cells.  Secondly,  the  twofold 
maturation-division  would  result,  not  in  halving,  but  in  quartering  the  original 
number  of  chromosomes.  I  prefer,  therefore,  to  follow  Boveri,  and  regard 
the  two  groups  of  four  grains  as  only  two  chromosomes,  this  number  being 
half  that  of  the  chromosomes  in  the  somatic  cells,  which  is  therefore  already 


known  also  as  the  Weismann  type,  as  Weismann  laid  great 
stress  upon  it,  although  he  did  so  chiefly  for  theoretical  reasons 
connected  with  his  theory  of  heredity.  At  the  maturation 
of  the  eggs  of  the  Copepods  among  Crustacea,  Kiickert  and  V. 
Haecker  observed  twelve  tetrads  (groups  of  four),  which,  they 
believed,  split  longitudinally  at  the  first  division,  and  trans- 
versely at  the  second,  which  would  then  be  a  reducing  division 
in  Weismann's  sense. 

Vom  Kath  described  similar  phenomena  occurring  at  the 
maturation  of  the  egg  of  the  mole-cricket  (Gryllotalpa),  but, 
according  to  Korschelt  and  Heider  (p.  586),  it  is  still  uncertain 
whether  the  second  division  in  this  case  -is  really  a  reducing 
division.  With  regard  to  many  other  insects  also  in  the 
last  few  years  the  post-reduction  division  has  been  frequently 
called  in  question,  and  it  must  be  observed  that  the  interpre- 
tation of  the  second  division  as  a  reducing  division  is  still  a 
moot  point ;  for  instance,  the  same  microscopical  observations 
of  the  maturation  of  the  sperm-cell  in  Orthoptera  led  McClung 
in  1900 !  to  declare  the  division  to  be  reducing,  and  de  Sinety 
(1901  and  1902)  to  pronounce  it  to  be  a  double  longitudinal 
splitting  of  the  eumitotic  type. 

t  The  kind  of  reducing  division  that  I  have  termed  pre- 
reduction,  in  which  the  reducing  precedes  the  equal  division, 
has  been  described  as  occurring  both  in  spermatogenesis  and 
oogenesis  of  animals  of  widely  different  types.  It  was  dis- 
covered by  Korschelt,  who  observed  it  at  the  maturation  of 
the  egg  of  the  annelid  Orphryotrocha  puerilis,  and  has  been 
called  after  him  the  Korschelt  type.  Henking  and  Paulmier 
say  that  this  kind  of  maturation- division  occurs  in  many 
species  of  Hemiptera,  and  Montgomery  has  traced  it  in  other 
Hemiptera  and  in  the  very  obscure  Peripatus.  On  the  other 
hand,  Gross2  declares  not  the  first,  but  the  second,  division 
to  be  reducing  in  the  maturation  of  the  sperm-cells  of  the 
Syromastes  marginatus,  so  that  this  bug  would  seem  to  supply 
an  instance  of  post-reduction  rather  than  of  pre-reduction 

1  See  also  McClung's  more  recent  work,  '  The  Spermatocyte   divisions  of 
the  Locustidae '  (Kansas  Univ.  Science  Bullet.,  I,  1902,  No.  8,  pp.  185-231, 
with  four  plates). 

2  '  Ein  Beitrag  zur  Spermatogenese  der  Hemipteren  '  ( Verhandl.  der  Deut- 
schen  Zoolog.  Gesellsch.,  1904,  pp.  180-190). 


E.  B.  Wilson's  latest  investigations  regarding  the  matura- 
tion-divisions of  germinal  vesicles  among  Hemiptera l  seem  to 
show  that  the  question  of  longitudinal  or  transverse  divisions 
has  lost  its  primary  importance,  because  the  chromosomes 
separating  at  the  reducing  division  were  originally  distinct, 
and  were  only  temporarily  united  during  an  intermediate 
synapsis  stage.3 

Montgomery  and  several  other  authors  ascribe  parti- 
cular importance  to  the  copulation  of  chromosomes  during 
synapsis  as  facilitating  the  interchange  of  qualities  be- 
tween the  chromosomes  of  the  male  and  female  parents 

Lastly,  bireduction  division,  in  which  both  maturation- 
divisions  of  the  germ-cells  are  reducing,  has  been  described 
by  Julin  as  occurring  at  the  maturation  of  the  egg  of  an  Ascidian 
(Styelopsis) ,  and  by  Wilcox  at  that  of  the  spermatozoon  of  a 
grasshopper  (Caloptenus),  &c.  The  remark  that  the  interpre- 
tation to  be  assigned  to  the  microscopical  observations  is  by 
no  means  certain,  applies  to  this  kind  of  division  even  more 
than  to  the  others. 

Some  idea  of  the  difficulties  which  the  student  engaged 
in  this  department  of  research  has  to  encounter,  may  be 
formed  from  the  fact  that  the  chief  supporters  of  the  various 
division  theories  have  repeatedly  changed  their  minds,  and 
have  assigned  to  their  observations  now  one  interpretation 
and  now  another.  I  may  refer  particularly  to  Boveri  and 
Strasburger  in  this  respect. 

As  we  have  seen  (p.  112),  Boveri  first  described  the  eumitotic 
type  of  maturation-division,  which  is  called  by  his  name,  and  in 
which  both  divisions  are  equal  and  longitudinal,  the  reduction 
in  the  number  of  chromosomes  having  taken  place  before  the 
division  ;  in  1903,4  however,  he  acknowledged  that  in  a  number 
of  instances  an  actual  reducing  division  takes  place,  '  though 
not  precisely  in  Weismann's  sense.'  Now  he  thinks  that  only  the 

1  *  Studies   on  Chromosomes '   (Journal  of  Experimental  Zoology,   II,  III, 
1905,  1906).     Cf.  also  p.  110,  note  2. 

2  On  the  subject  of  this  stage  see  Pantel  and  de  Sine~ty,  '  Les  cellules  de  la 
lignee  male  chez  le  Notonecta  glauca  '  (La  Cellule,  XXIII,  1906,  fasc.  I,  pp. 
89-303),  pp.  lll,&c. 

3  See  0.  Hertwig,  Allgemeine  Biologie,  pp.  291,  292. 

4  Boveri,  '  Uber  die  Konstitution  der  chromatischen  Kernsubstanz  '  (Ver- 
handl.  der  Deutschen  Zoolog.  Gesellsch.,  1903,  pp.  10-32),  p.  27. 

i  2 


first  division  is  longitudinal,  and  he  believes  the  second  to  be 
transverse,  effecting  a  reduction  in  the  number  of  chromosomes. 
If  this  is  true,  we  have  post-reduction  division,  approximating 
to  the  Weismann  type. 

In  1904  Strasburger,1  the  botanist,  abandoned  his  earlier 
opinions  regarding  the  eumitotic  type  of  maturation.  His 
most  recent  investigations  of  the  pollen-mother-cells  of  Galtonia 
show  the  first  of  the  two  maturation-divisions  of  the  chromo- 
somes to  be  transverse,  resulting  in  a  reduction  of  their  number  ; 
the  second,  on  the  contrary,  appears  to  be  a  longitudinal  or 
equal  division.  In  1904,  therefore,  Strasburger,  it  would  seem, 
upheld,  instead  of  the  eumitotic  type,  the  pseudomitotic,  in 
the  form  of  a  pre-reduction  division,  corresponding  to  the 
Korschelt  type.  But  we  should  have  almost  as  much  justifi- 
cation for  speaking  of  post-reduction  in  this  case  ;  for,  as 
Strasburger  expressly  states,  the  longitudinal  division,  which  is 
actually  the  second  in  order  of  occurrence,  is  anticipated  by 
a  longitudinal  splitting  of  the  chromosomes,  which  precedes 
the  first  transverse  division.  In  1905,  however,  Strasburger 
returned  to  his  earlier  opinion  regarding  the  eumitotic  type 
of  maturation-divisions,2  and  he  now  again  maintains  that 
both  divisions  are  longitudinal  and  equal,  and  that  the  real 
reduction  in  the  number  of  chromosomes  precedes  them. 
He  agrees,  therefore,  now  with  Abbe  V.  Gregoire,  who  expressed 
similar  views  in  1905.3 

The  theory  of  eumitotic  maturation-division  seems,  there- 
fore, to  have  triumphed  over  that  of  pseudomitotic.4  Whether 
inthechromatin  skein  or  spireme,  formed  before  the  maturation- 
divisions  take  place,  the  individual  chromosomes  are  joined 
longitudinally  or  by  their  apex,  is  a  question  raised  by  Boveri 
in  1903,  and  discussed  by  Gregoire,  Strasburger,  Schreiner5 

1  Strasburger,  '  Uber  Beduktionsteilung  '  (Sitzungsber.  der  Berl.  Akademie 
der  Wissensch.,  XIV,  1904,  pp.  587-614). 

1st,,    JLJLHsU/rl't'tVy    ^VJ-JJ-J-j     J.  t/VcJj    i  Cl/1  U   J.y     KMr*     x — *  A  /* 

V.  Gregoire,  '  Les  resultats  acquis  sur  les  cineses  de  maturation  dans  lea 
regnes ' :  I.  memoire  :  Revue  critique  de  la  litterature  (La  Cellule.  XXII, 

2  Strasburger,  '  Typische  und  allotypische  Kernteilung '  (Jahrb.  fur  wissen- 
schaftl  Botanik,  XLII.  1905,  Part  I,  pp.  1-71). 

3  V. 

1905,  fasc.  2,  pp.  221-374). 

4  Cf .  J.  Marechal,  '  Uber  die  morphologische  Entwicklung  der  Chromosomen 
im  Selachierei  und  Teleostierei '  (Anatom.  Anzeiger,  XXV,  1904,  pp.  383-398 
and  XXVI,  1905,  pp.  641-652). 

5  A.  and  K.  E.  Schreiner,  '  Neue  Studien  iiber  die  Chromatinreifung  der 
Geschlechtszellen  '  (Archives  de  Biohgie,  XXII,  1906,  fasc.  I,  pp.  1-69). 


and  Bonnevie,1  but  we  cannot  consider  it  fully  now.  The 
first  view  is  probably  the  correct  one.  I  may  remark  inci- 
dentally that  almost  all  the  recent  results  of  the  examination 
of  chromosomes  tend  to  confirm  Boveri's  theory  of  their 
'  individuality.'  But  I  shall  recur  to  this  theory  in  the  ninth 
section  of  this  chapter. 

J.  Gross 3  has  recently  summed  up  the  results  of  his  inves- 
tigations into  the  maturation-divisions  of  the  germ-cells  in  the 
following  sentence  :  '  The  most  important  results  of  cytological 
research  into  the  problem  of  reduction  in  the  last  few  years  seem 
to  me  to  be  two  :  it  has  been  demonstrated  that  a  real,  qualita- 
tive reduction  actually  takes  place,  and  it  has  been  found  that  a 
conjugation  of  the  chromosomes  of  both  parents  as  a  rule 
precedes  the  maturation-divisions.' 

I  have  already  dwelt  too  long  upon  the  various  theories 
connected  with  the  maturation  of  the  germ-cells.  The  accom- 
panying diagrams  will  enable  the  reader  to  form  some  idea 
of  the  maturation  of  the  egg-cell  and  of  the  formation  of  the 
polar  bodies  ;  they  represent  the  particular  kind  of  division 
that  I  have  termed  post-reduction.  It  must,  however,  be 
observed  that  these  are  merely  diagrams,  and  do  not  represent 
the  actual  process  ;  they  have  been  designed  to  show,  in  the 
simplest  way  possible,  the  first  division  as  equal,  and  the 
second  as  reducing. 

Let  us  assume  the  primary  oocyte  to  have  four  chromosomes 
in  its  nucleus  before  the  process  of  division  begins.  The  first 
stage  in  the  process  is  that  the  germ-nucleus  or  vesicle  moves 
towards  the  periphery  of  the  cell  (fig.  17).  Then  the  chromo- 
somes of  the  nucleus  arrange  themselves  in  the  manner  de- 
scribed in  Chapter  V  (p.  94),  so  as  to  form  an  equatorial  plate 
or  crown  in  the  middle  of  an  achromatic  nuclear  spindle 
(fig.  18)  ;  they  split  longitudinally,  and  the  daughter-chromo- 
somes withdraw  to  the  poles  of  the  nuclear  spindle  (fig.  19). 
This  first  nuclear  division  is  an  equation  or  equal  division  of 
the  ordinary  kind,  not  a  reducing  division.  The  upper  group 
of  four  chromosomes  with  the  centrosome  of  the  egg-cell 

1  '  Untersuchungen  iiber  Keimzellen  :  I.  Beobachtungen  an  den  Keimzellen 
von  Enteroxenos  Oestergreni '  (  Jenaische  Zeitschr.  fur  Naturwissensch.,  XLI, 
1906,  part  2,  pp.  229-428). 

2  '  Uber  einige  Beziehungen  zwischen  Vererbung  und  Variation '  (Biolog. 
Zentralblatt,  1906,  Nos.  13-15,  &c.,  p.  396). 



belonging  to  them  is  now  forced  against  the  periphery  of  the 
cell,  until  it  finally  passes  out  of  the  cell,  surrounded  by  a 
small  quantity  of  protoplasm  (fig.  20).  This  forms  the  first 
polar  body  (rl  in  fig.  20).  Meantime,  a  fresh  nuclear  spindle 
forms  immediately  round  the  four  chromosomes  left  in  the 
egg-nucleus  (fig.  20)  ;  but  this  time  there  is  no  longitudinal 

FIG.  17. 

FIG.  18. 

FIG.  19. 

FIG.  20. 

FIG.  21. 

FIG.  22. 

FIGS.  17-22. — Diagrams  representing  the  maturation-divisions  of  the  egg-cell. 
r^=  first  polar  bo<j|T;  r"—  second  polar  body;  vk~ female  pronuclcus. 

splitting  of  the  chromosomes.  They  arrange  themselves  in 
pairs  (fig.  21)  ;  the  upper  pair  approach  the  periphery  of 
the  cell,  and  are  expelled  from  it  with  a  particle  of  protoplasm, 
and  so  form  the  second  polar  body  (r-  in  fig.  22).  This 
second  division  was  reducing,  for  the  nucleus  of  the  egg-cell, 
which  now  resumes  its  original  shape,  and  at  this  stage  is 
called  the  female  pronucleus  (v~k  in  fig.  22),  now  has  only  two 
chromosomes  instead  of  four.  If,  in  the  meantime,  the  first 
polar  body  has  again  divided  (n  in  figs.  21  and  22),  the 


result  of  the  two  maturation-divisions  of  the  egg-cell  has  been 
the  production  of  one  large  and  three  small  cells,  of  which  only 
the  first,  the  egg-cell  prepared  for  fertilisation,  is  of  interest 
for  us.1 


(See  Plate  I) 

Let  us  now  turn  to  the  process  of  fertilisation  in  its  normal 
form  in  animal  ova,  as  microscopical  research  has  revealed  it 
to  us.  0.  Hertwig  was  the  first  to  succeed,  in  1875,  in  lifting 
the  veil  that  for  so  many  thousands  of  years  had  rested  over 
these  phenomena.  In  the  course  of  observations  on  the 
eggs  of  the  sea-urchin  (Echinus),  he  saw  that  during  fertilisation 
a  thread-like  sperm-cell  passes  into  the  ovum  ;  the  head  of 
the  sperm-cell  changes  into  a  so-called  male  pronucleus,  and 
unites  with  the  nucleus  of  the  ovum,  or  female  pronucleus. 
This  union  of  nuclei  results  in  the  normal  process  of  fertilisation, 
for  it  gives  rise  to  the  cleavage-nucleus  of  the  fertilised  ovum, 
which  at  once  begins  to  divide  by  means  of  the  nuclear 
spindle  of  the  cleavage-nucleus,  so  forming  the  first  pair 
of  cleavage-corpuscles,  or  blastomeres,  from  whose  further 
divisions  all  the  tissues  and  organs  of  the  new  individual 
are  produced. 

At  first  sight  the  process  of  fertilisation  thus  described 
seems  very  simple,  but  it  becomes  very  complex  by  reason 
of  the  vast  varieties  in  its  details,  in  the  case  of  different  plants 
and  animals.  Moreover,  very  various  opinions  still  prevail  as 
to  the  parts  played  by  the  cell-nucleus,  the  centrosome,  and  the 
egg-plasm  respectively  in  the  work  of  fertilisation.  Korschelt 
and  Heider  devote  over  one  hundred  pages  to  a  description 
of  these  phenomena  in  their  '  Vergleichende  Entwicklungs- 
geschichte  der  wirbellosen  Tiere '  (Allgemeiner  Teil,  pp.  628,  &c.). 
I  must  obviously  limit  myself  to  what  is  absolutely  necessary 

1  For  the  subsequent  history  of  the  polar  bodies  (globules  polaires)  and  their 
importance,  see  Korschelt  and  Heider,  Lehrbuch  der  vergl.  Entwicklungsgesch., 
pp.  549,  &c.  They  discuss  Petrunkewitsch's  theory  that  the  polar  bodies 
continue  to  exist  and  supply  the  material  for  the  germinal  glands  of  the 
future  embryo.  But  nothing  is  known  with  certainty  on  the  subject- 


in  order  to  enable  my  readers  to  form  some  idea  of  the  essential 
processes  of  fertilisation  and  heredity. 

Although  the  ovum  of  the  Echinus  measures  only  ^  mm. 
in  diameter,  it  is,  like  all  other  ova,  of  enormous  size  in  com- 
parison with  the  spermatozoon — and  this  is  especially  true 
in  the  case  of  eggs  containing  much  yolk.  Such  eggs  have 
stored  up  in  their  egg-plasm  a  considerable  quantity  of 
nutritive  matter,  which  is  used  in  the  development  of  the 
future  embryo.  The  sperm-cells,  on  the  contrary,  are  some  of 
the  smallest  cells  occurring  in  living  organisms,1  for  their 
sole  task  is  to  penetrate  the  ovum  and  fertilise  it.  For  this 
reason  the  protoplasm  that  constitutes  the  cell-body  is  generally 
only  a  thread-like  flagellum,  which  serves  as  an  organ  of 
locomotion,  and  the  thickened  head  is  the  nucleus  of  the 
sperm-cell ;  between  head  and  tail  is  the  so-called  middle- 
piece  containing  the  centrosome  of  the  sperm-cell. 

In  spite  of  the  extraordinary  difference  in  size  and  shape 
between  the  ovum  and  the  spermatozoon,  their  nuclei  are  so 
far  of  absolutely  equal  value,  for  they  contain  the  same  number 
of  chromosomes.  Both  the  male  and  the  female  pronuclei 
contain  half  the  number  of  chromosomes  found  in  the  somatic 
cells  of  the  same  species.  This  fact,  to  which  I  referred  in 
speaking  of  the  maturation-divisions  of  the  germ-cells,  is  of 
great  importance  in  our  consideration  of  fertilisation  and 

The  union  of  the  male  and  female  pronuclei  to  form  the 
cleavage -nucleus  of  the  fertilised  ovum  does  not  necessarily 
involve  a  real  fusion  of  the  nuclei ;  on  the  contrary,  in  many 
cases  the  nuclei  with  their  chromosomes  remain  distinct  from 
one  another,  though  they  take  up  their  positions  close  together, 
so  as  to  form  a  common  cleavage-spindle.  We  may  follow 
Korschelt  and  Heider  (p.  682)  in  distinguishing  two  chief  types 
of  fertilisation.  The  first  is  the  so-called  Echinus-type,  deriving 
its  name  from  the  sea-urchin  (Echinus),  in  which  it  was  first 
observed  and  described  by  0.  Hertwig  (1875-1878).  In  this 
type  the  two  pronuclei  actually  fuse  together  to  form  one 
resting  cleavage-nucleus,  which  does  not  begin  to  divide  until 
the  fusion  is  complete.  It  should  be  noticed,  however,  that 

1  In  mammals  they  often  measure  (without  the  tail  filament)  only  0*003  mm. 
See  R.  Hertwig,  Lehrbuch  der  Zoologie,  1905,  p.  49  (Eng.  trans,  p.  60). 


the  chromosomes  of  the  two  pronuclei  do  not  fuse  together, 
but  come  into  close  juxtaposition.  The  second  type  is  the 
Ascaris-type,  deriving  its  name  from  the  maw-worm  of  the 
horse  (Ascaris  megalocephala),  in  which  it  was  observed  by  E. 
van  Beneden  in  1883  ; l  in  it  the  two  pronuclei  remain  indepen- 
dent, but  take  up  their  position  close  together,  so  as  to  produce 
the  first  cleavage -spindle  in  common.  Having  produced  it, 
they  break  up,  and  distribute  their  chromosomes  by  longitu- 
dinal division  to  the  two  daughter-nuclei.  Many  instances 
of  both  types  of  union  occur  in  the  animal  kingdom,  in  very 
various  families  and  classes,  and  also  in  closely  related  species  ; 
in  fact  Boveri  (1890)  and  Klinckowstrom  (1897)  have  found 
them  even  within  one  and  the  same  species. 

I  have  chosen  the  second  type  to  illustrate  the  normal 
phenomena  of  fertilisation,  because  it  has  the  advantage  ,of 
showing  more  clearly  how  the  paternal  and  maternal  chromo- 
somes are  evenly  distributed  at  the  cleavage  of  the  fertilised 
ovum.  In  a  lecture  on  the  subject  of  fertilisation  ('  Das 
Problem  der  Befruchtigung,'  Jena,  1902),  Boveri  sketched  the 
process  on  the  lines  of  the  Ascaris-type,  illustrating  it  by 
diagrams,  which  are  reproduced  on  Plate  I,  figs.  1-7. 2 

The  egg-nucleus  is  coloured  blue  and  the  sperm-nucleus 
red,  in  order  to  make  it  easy  to  distinguish  the  two  nuclei  and 
the  chromosomes  of  the  cleavage-spindle  proceeding  from  them. 

The  nucleus  of  the  mature  egg-cell,  which  after  the  matura- 
tion-divisions is  called  the  female  pronucleus,  moves  from  the 
excentric  position,  occupied  during  the  formation  of  the 
polar  bodies,  back  into  the  centre  of  the  cell  (Plate  I,  fig.  1). 
Meantime  a  spermatozoon  has  made  its  way  into  the  ovum  (at 
the  top  of  fig.  I).3  Only  its  head  and  middle-piece,  however, 

1  This  type  was  perhaps  observed  by  0.  Hertwig  between  1875  and  1878  as 
occurring  in  Mitrocoma  and  Aequorea  (Korschelt  and  Heider,  p.  681). 

2  I  say  '  on  the  lines  of  the  Ascaris-type,'  because  in  many  details  this  sketch 
is  at  variance  with  actual  observations  made  by  E.  van  Beneden,  0.  Hertwig, 
Carnoy,  Boveri,    &c.,    on  Ascaris  megalocephala  var.  bivalens.     It  should  be 
noticed  particularly  that  in  Ascaris  the  spermatozoon  does  not  lose  a  tail, 
but  the  whole  sperm-cell,  which  in  this  case  is  conical,  passes  into  the  egg- 
plasm.     Cf.  also  E.  Korschelt,     *  Uber  Morphologic  und  Genese  abweichend 
gestalteter  Spermatozoon '  (Verhandl  der  Deutschen  Zoolog.  Gesellsch.,  1906, 
pp.  73-82). 

3  Circumstances  vary  greatly  in   different   cases.     In   some   animals   the 
maturation-divisions  of  the  egg  precede  the  entrance  of  the  spermatozoon, 
in  others  they  are  simultaneous  with  or  subsequent  to  it.     Cf.  Korschelt  and 
Heider,  pp.  630-632. 


really  enter  it ;  the  tail  filament,  representing  the  protoplasmic 
body  of  the  sperm-cell,  is  generally  thrown  off,  or  it  is  quickly 
resolved  in  the  protoplasm  of  the  egg-cell.  The  head  and 
middle-piece  of  the  spermatozoon  rotate  through  180°,  so 
that  the  middle-piece,  which  was  previously  behind  the  sperm- 
head,  is  now  in  front  of  it ;  the  spermato-centrosome,  or  cen- 
trosome  of  the  sperm-cell,  contained  in  the  middle-piece,  now 
becomes  visible,  and  sends  out  a  ring  of  protoplasmic  rays 
(fig.  2),  the  so-called  '  sperm-aster,'  which  is  here  represented 
as  small,  although  it  often  stretches  over  the  greater  part  of 
the  egg.  A  very  remarkable  transformation  of  the  sperm- 
head  now  begins.  It  swells  up — in  consequence,  as  Y.  Delage 
thinks,  of  taking  in  water  from  the  egg-plasm — and,  as  it  swells, 
it  reveals  its  nuclear  character  by  forming  a  chromatin  frame- 
work (Plate  I,  figs.  3  and  4),  until  finally  it  appears  as  a  male 
pronucleus  (fig.  5),  exactly  equivalent  to  the  female.  Mean- 
time the  spermato-centrosome  has  undergone  a  series  of 
further  modifications.  It  divides  (Plate  I,  fig.  3) ;  the  two 
half-centrosomes  take  up  a  position  on  either  side  of  the  two 
nuclei  (fig.  4)  and  develop  their  astrospheres  (fig.  5).  The 
chromatin  substance  of  the  two  pronuclei,  now  in  close  proxi- 
mity, next  proceeds  to  transform  its  chromatin  framework, 
in  readiness  for  the  first  cleavage  of  the  egg-cell.  Each  pro- 
nucleus  develops  the  same  number  of  chromatin  loops,  which 
usually  resemble  one  another  exactly  in  size  and  shape.  In 
the  diagram  (fig.  6),  which  might  be  taken  as  representing  the 
fertilisation  of  the  maw-worm  of  the  horse,  Ascaris  megalo- 
cephala  var.  bivalens,  each  pronucleus  contains  two  chromatin 
loops  or  chromosomes,  i.e.  half  the  number  contained  by  the 
somatic  cells  of  the  same  animal.  The  cleavage-spindle  is  next 
formed  ;  it  gives  rise  to  the  first  division  of  the  fertilised  egg- 
cell,  and  .so  to  the  first  stage  in  the  development  of  the  future 

Each  of  the  two  chromosomes  in  the  parent  nuclei  splits 
lengthwise  into  two  parts,  which  arrange  themselves  in  the 
middle  of  the  nuclear  spindle  formed  by  the  centrosomes 
(fig.  7).  Then  the  four  daughter-chromosomes  on  the  left, 
two  being  paternal  and  two  maternal  in  origin,  move  to  the 
left  pole  of  the  spindle  ;  the  corresponding  four  on  the  right 
move  to  the  right  pole  of  the  spindle,  and  at  the  two  poles  they 


give  rise  to  the  two  daughter-nuclei  of  the  first  cleavage-cells 
(blastomeres)  of  the  embryo.  Thus  each  of  the  first  two  daughter- 
cells  contains  four  chromosomes  in  its  nucleus,  two  from  the 
father  and  two  from  the  mother.  Hence  it  comes  about  that 
each  of  the  cells  in  the  embryo,  which  are  produced  by  continued 
indirect  karyokinesis  from  the  fertilised  ovum,  contains  an 
equal  number  of  paternal  and  maternal  chromosomes,  and  the 
total  number  is  equal  to  that  of  the  chromosomes  in  the 
somatic  cells  of  the  parents,  and  double  that  contained  in 
either  the  male  or  female  pronucleus.  It  would  seem,  therefore, 
that  by  this  process  a  precisely  equivalent  transmission  of  the 
nuclear  elements  of  both  parents  is  secured  to  their  offspring. 

We  must  here  refer  to  an  observation,  made  originally  by 
Boveri  in  1887  l  and  confirmed  by  subsequent  study  of  Ascaris 
megalocephala,  which,  whilst,  to  some  extent,  modifying  the 
account  just  given,  lends  it  additional  weight  in  its  bearing 
upon  the  question  of  transmission.  In  Ascaris;  in  all  the 
cleavages  from  the  two-cell  stage  onwards,  the  cells  of  the 
germinal  area  of  the  embryo  present  characteristics  in  their 
nuclei  and  processes  of  karyokinesis  distinguishing  them  from 
the  somatic  cells  of  the  same  embryo.  Only  the  cleavage- 
granules  destined  to  give  rise  to  the  germ-cells  preserve  the 
original  chromosomes,  which  they  receive  from  the  fertilised 
egg-cell,  in  unaltered  form  ;  the  cleavage-granules  destined  to 
produce  the  somatic  cells,  as  soon  as  they  begin  to  divide, 
reject  the  thickened  ends  of  the  chromosomes,  and  the  rest  of 
the  chromatin  loop  breaks  up  into  a  number  of  smaller  pieces, 
that  subsequently  reappear.  Boveri  called  this  phenomenon 
'  chromatin  diminution,'  and  it  seems  to  show  that  only  in  the 
germ-areas  is  the  continuity  of  the  germ-plasm  fully  main- 
tained, whilst  many  divergencies  may  occur  in  the  tracts  of 
somatic  cells.3 

It  is  a  fact  that  individuals,  born  of  the  same  parents, 
differ  to  a  certain  extent  both  from  their  parents  and  from  one 
another,  and  it  is  no  less  true  that  the  qualities  of  grand- 
parents or  of  their  collateral  relatives,  latent  in  the  generation 

1  Cf.  Korschelt  and  Heider,  pp.  151,  152. 

2  For  further  evidence  in  support  of  this  theory,  see  Boveri,  '  Uber  die 
Konstitution   der    chromatischen   Kernsubstanz,'    pp.  18-20   (VerhandL   der 
Deutschen  Zoolog.  Gesellsch.,  Wurzburg,  1903,  pp.  10-33).     Cf.  also  0.  Hertwig, 
Attgem.  Biologic,  1906,  pp.  199-201). 



next  in  succession,  reappear  suddenly  in  the  grandchildren. 
Boveri's  microscopical  observations,  to  which  we  have  referred, 
may  be  taken  as  corroborating  the  theory  that  the  chromatin 
elements  of  the  nucleus  are  the  means  of  transmitting  heredi- 
tary properties.  There  is,  therefore,  actual  evidence  in  support 
of  the  theory  held  by  Eoux,  Strasburger,  0.  and  K.  Hertwig, 
Weismann,  Kolliker,  Boveri,  &c.,  that  in  the  chromosomes  of 

FIG.  23. — Transverse  section  of  the  blastula  stage  of  an  embryo  of 
Ascaris  megalocephala  var.  bivalens. 

the   nucleus  we  may  discover  the  real  substance  of  heredity, 
which  Nageli  calls  idioplasm. 

In  order  to  illustrate  the  differentiation  of  the  germ-cell 
area  from  the  somatic-cell  area  in  the  case  of  Ascaris  megalo- 
cephala var.  bivalens,  I  give,  in  fig.  23,  an  exact  microscopical 
reproduction  of  a  transverse  section  of  the  embryo  of  this 
creature  at  the  blastula  stage.1 

1  The  figure  is  taken  from  a  long  series  of  sections,  stained  with  Heidenhain's 
iron-haematoxylin,  showing  the  maturation-divisions  and  the  processes  of 
fertilisation  and  development  in  Ascaris  megalocephala.  The  series  was 
prepared  by  my  colleague,  K.  Frank,  S.J.,  under  Heider's  direction.  In  the 
original  the  centrosomes  at  the  two  ends  of  the  cleavage -spindle  in  cells  c  and  d 
can  be  seen  more  plainly  than  in  the  reproduction ;  they  seem  to  be  little 
circular  formations  marked  off  from  the  surrounding  plasmic  rays. 


The  two  uppermost  cells,  a  and  b,  are  two  somatic  cells 
with  resting  nuclei,  in  each  of  which  two  dark  spots,  nucleoli, 
can  be  plainly  seen.  The  two  middle  cells,  c  and  d,  are  like- 
wise two  somatic  cells,  but  they  are  still  in  the  act  of  mitosis  ; 
the  fine  chromatin  rods,  still  grouped  about  the  equatorial 
plate  in  the  centre  of  the  plainly  visible  achromatic  nuclear 
spindle,  are  actually  in  process  of  division.  Also  the  centro- 
somes  with  their  astrospheres  at  the  two  poles  of  the  spindle 
are  shown  very  beautifully.  Hence  this  illustration  serves 
to  supplement  the  formal  diagrammatic  representation  given 
in  Chapter  V  of  the  process  of  indirect  nuclear  division  (see 
p.  95).  The  lowest  cell,  e,  with  its  four  large  chromatin-loops, 
represents,  according  to  Boveri,  one  of  the  germ-cells  in  the 
embryo.  There  is  a  great  difference  between  the  chromosomes 
in  it  and  those  in  the  somatic  cells,  and  the  fact  that  the  future 
germ-cells  contain  much  more  chromatin  than  the  somatic 
cells,  is  an  argument  in  favour  of  the  theory  that  the  chromo- 
somes of  the  nucleus  are  the  bearers  of  heredity.  We  do  not 
yet  know  how  the  normal  number  of  four  chromosomes,  which 
subsequently  are  present  in  the  somatic  cells  of  Ascaris,  arises 
out  of  the  numerous  chromatin  rods  of  the  somatic  cells  c  and  d. 

Let  us  now  refer  again  to  the  account  already  given  of 
the  process  of  fertilisation  in  the  Ascaris-type.  This,  and  the 
EMnus-iype,  which  differs  from  it  by  the  formation  of  one 
cleavage-nucleus,  both  show  us  that,  in  the  first  place,  fertilisa- 
tion leads  to  the  beginning  of  the  embryonic  development  of  a 
new  individual,  because  it  causes  the  cells  to  divide  ;  in  the 
second  place,  it  restores  the  normal  number  of  chromosomes 
for  all  the  somatic  cells  of  the  new  individual ;  and  lastly 
it  distributes  to  every  cell  of  the  embryo,  as  an  inheritance,  an 
equal  number  of  chromosomes  derived  from  each  parent. 

The  last  two  facts  taken  in  conjunction  show  the  bearing 
of  fertilisation  upon  heredity  ;  the  first  shows  its  bearing  upon 
germinal  development. 

As  I  shall  have  to  discuss  the  theoretical  value  of  these 
phenomena  at  the  close  of  this  chapter,  it  must  suffice  for 
the  present  thus  briefly  to  indicate  the  twofold  object  of 

I      Before   passing   on   to   other   points   connected  with   the 
problem  of  fertilisation,  I  must  once  more  refer  to  the  normal 


process  as  already  described  and  as  illustrated  by  Boveri's 
diagrams  (Plate  I,  figs.  1-7).  We  may  ask  :  '  What  is  it 
in  this  case  that  gives  rise  to  the  formation  of  the  cleavage- 
spindle,  and  thus  to  the  first  division  of  the  ovum,  which  con- 
stitutes the  starting  point  in  the  development  of  the  embryo  ?  ' 
The  impulse  proceeds  from  the  male  centrosome,  which  pene- 
trates into  the  ovum  with  the  middle-piece  of  the  spermatozoon. 
In  the  course  of  the  preceding  maturation-divisions  the  centro- 
some of  the  egg-cell  either  is  lost  or  degenerates,  and  conse- 
quently, in  spite  of  possessing  a  great  quantity  of  nutritive 
plasm,  the  egg-cell  is  incapable  of  further  division,  for,  in  losing 
its  centrosome,  it  has  lost  its  kinoplasm,  as  Strasburger  calls 
it,  the  active  motorplasm  in  the  cell.  It  requires,  therefore,  a 
new  *  organ  of  division  '  before  it  can  proceed  to  embryonic 
development,  and  this  organ  of  division  is,  in  normal  fertilisa- 
tion, the  centrosome  of  the  sperm-nucleus.  Its  division  gives 
rise  to  the  two  centrosomes  (Plate  I,  figs.  2-6)  which  form  the 
poles  of  the. first  cleavage-spindle  (Plate  I,  fig.  7)  and  cause  the 
chromatin  loops  of  the  united  male  and  female  pronuclei  to  be 
distributed  evenly  between  the  first  two  cleavage-nuclei  of  the 
fertilised  ovum. 

This  account  of  the  process  of  fertilisation  was  first  given 
by  Boveri  in  1887  ; l  according  to  it,  the  impulse  giving  rise  to 
embryonic  development  is  not  supplied  by  the  union  of  the 
two  pronuclei,  but  is  the  primary  object  of  the  fertilisation 
caused  by  the  introduction  of  the  sperm-centrosome  into  the 
ovum.  The  union  of  the  pronuclei  is  the  secondary  object, 
and  produces  the  transmission  of  the  qualities  of  both  parents 
to  the  offspring,  but,  according  to  this  view,  it  is  only  a  result 
of  the  action  of  the  male  centrosome  upon  the  protoplasm  of 
the  female  egg-cell. 

As  Boveri  himself  is  careful  to  state,2  this  account  of  the 
process  of  fertilisation  is  not  universal  in  its  application  ; 
it  cannot  be  applied  to  all  forms  of  fertilisation  in  animals  and 
plants,  but  only  to  those  of  most  multicellular  animals  ;  3  for 

1  '  tiber  den  Anteil  des  Spermatozoons  an  der  Teilung  des  Eis  '  (Sitzungs- 
bericht  der  Gesellsch.  jiir  Morphol.  u.  Phys,,  Munich,  III). 

2  Das  Problem  der  Befruchtung,  pp.  23,  &c. 

a  According  to  Wheeler  the  centrosome  of  the  ovum  remains  in  Myzostoma, 
and  forms  the  poles  of  the  cleavage-spindle.  Cf.  Korschelt  and  Heider, 
p.  657. 


hitherto  no  centrosome  has  been  observed  at  the  fertilisation 
of  the  higher  kinds  of  plants,1  nor  at  the  conjugation  of  uni- 
cellular animals. 

In  natural  parthenogenesis  the  development  of  the  ovum 
takes  place  without  fertilisation  by  a  male  germ-cell,  and  so  no 
spermato-centrosome  occurs,  therefore  it  is  not  essential  to 
give  rise  to  the  embryonic  development  of  the  egg.  Eecent 
experiments  in  artificial  parthenogenesis  have  succeeded,  by 
means  of  various  mechanical,  thermal,  chemical  or  other 
stimuli,  in  causing  centrosomes  to  form,  and  the  subsequent 
cell-division  to  take  place,  in  the  unfertilised  eggs  of  animals, 
in  which,  under  normal  circumstances,  the  male  centrosome 
supplies  the  cell  with  the  means  of  division.  We  must  therefore 
be  careful,  even  in  the  normal  fertilisation  of  animal  ova,  not 
to  ascribe  to  the  spermato-centrosome  too  much  influence  in 
setting  up  embryonic  development  in  the  ovum. 

We  can  thus  appreciate  the  reasons  which  led  so  great  an 
authority  on  the  problem  of  fertilisation  as  B.  Hertwig  to  con- 
tent himself  with  the  simple  statement  that  '  the  essential 
feature  of  fertilisation  consists  in  the  union  of  egg-  and  sperm- 
nuclei  '  (Lehrbuch  der  Zoologie,'  p.  124  :  Eng.  trans,  p.  149). 


Under  normal  conditions  during  the  process  of  fertilisation 
only  one  sperm-cell  penetrates  an  animal  ovum,  although  there 
may  be  hundreds  in  its  immediate  neighbourhood.  In  many 
eggs  this  is  secured  by  the  construction  of  the  enclosing  mem- 
brane, which  allows  spermatozoa  to  enter  at  one  point  only. 
In  the  case  of  eggs  with  no  such  point  of  entrance  (micropyle) 
the  same  result  is  attained  in  another  way — a  vitelline  mem- 
brane forms  immediately  after  the  entrance  of  one  spermatozoon, 
excluding  all  others.  If  the  reacting  power  of  the  egg  be 
weakened  by  means  of  strychnine,  or  other  poison,  so  that 
it  admits  several  spermatozoa,  a  normal  development  never 
results  ;  the  numerous  centrosomes  carried  into  the  egg  give 
rise  to  the  formation  of  karyokinetic  figures  with  several  poles, 
or  of  very  large  nuclei  which  divide  irregularly  and  lead  to  an 

1  Cf.  Chapter  V,  p.  99. 


abnormal  process  of  cleavage  and  to  the  speedy  death  of  the 
embryo.  Hence  Boveri  was  right  in  stating  emphatically 
in  1902  that  the  entrance  of  two  spermatozoa  ruins  a  perfectly 
normal  egg.  The  explanation  of  this  fact  is  that  the  intro- 
duction of  several  centres  of  division  into  the  egg  hinders  its 
normal  development. 

In  many  animals,  however,  exceptional  cases  have  been 
observed  when  several  sperm-cells  have  entered  one  egg  under 
normal  conditions.  (Gerard,  1901.)  But,  when  this  occurs, 
only  one  sperm-nucleus  unites  with  the  egg-nucleus,  and  the 
rest  are  absorbed  by  the  egg-plasm.  In  1902  Boveri 1  ob- 
served these  processes  in  sea-urchins'  eggs,  fertilised  with 
two  spermatozoa,  and  he  applied  the  results  of  his  observations 
very  ingeniously  to  his  investigations  into  the  nature  of  the 
nucleus  and  the  importance  of  the  chromosomes. 

We  must  distinguish  the  above-mentioned  pathological 
superfecundation  from  what  is  called  physiological  polyspermy, 
which  recent  research  has  proved  to  occur  in  many  kinds  of 
animals.  In  this  case  also  only  one  sperm-nucleus  unites  with 
the  egg-nucleus  to  form  the  first  cleavage-spindle,  but,  as 
Kiickert,  Oppel,  Samassa  (1895),  and  Nicolas  (1900)  have  ob- 
served, especially  in  the  eggs  of  Selachii  and  reptiles,  only  a 
few  of  the  other  nuclei  perish — many  of  them  are  transformed 
into  the  so-called  merocytes  or  yolk-nuclei  of  the  embryo  ;  not 
much  is  known  with  certainty  about  their  subsequent  fate,  but 
they  are  supposed  to  be  connected  with  the  vegetative  functions 
of  the  egg,  and  to  expedite  the  division  of  the  abundant  vitelline 

Closely  related  to  physiological  polyspermy  among  animals 
is  double-fertilisation,  an  interesting  phenomenon  occurring  in 
Angiosperms  among  the  higher  plants.  A  good  deal  of  light 
has  been  thrown  on  this  subject  and  on  its  biological  signifi- 
cance by  Nawaschin  (1898),  Guignard  (1899  and  1901),  and 
Strasburger  (1900).3 

In  this  process  two  sperm-nuclei  penetrate  into  the  embryo- 

1  *  tiber  mehrpolige  Mitosen  als  Mittel  zur  Analyse  des  Zellkerns     ( Ver- 
handl,  der  physikalisch-medizinischen  Oesellsch.,  Wiirzburg,  XXXV,  pp.  67-90). 

2  For  a  good  summary  of  works  published  before  1900,  and  dealing  with 
the  phenomena  of  double- fertilisation,  see  G.  Richen,  S.J.,  in  Natur  und  Offen- 
barung,  1900,  pp.  561,  &c.     Cf.  also  Korschelt  and  Heider,  Lehrbuch  der  vergl. 
Entwicklungsgeschichte,  p.  696. 


sac,  one  of  which  unites  as  the  male  pronucleus  with  the  egg- 
nucleus,  thus  forming  the  cleavage-nucleus  of  the  mother-cell 
of  the  embryo.  The  other  amalgamates  with  the  secondary 
nucleus  of  the  embryo-sac  (formed  by  the  union  of  the  two 
polar  cells),  or  in  some  cases  with  one  of  the  polar  cells 
before  their  union,  and  thus  produces  the  nucleus  of  the 
mother-cell  of  the  endosperm,  which  has  to  supply  nourish- 
ment to  the  embryo.  It  is  a  remarkable  fact  that  one  of  the 
two  sperm-nuclei  has  a  generative,  and  the  other  a  vegetative 
function  to  discharge. 

This  double  fertilisation  in  Angiosperms  is  of  importance 
in  explaining  some  mysterious  phenomena  in  heredity,  the 
so-called  xenia.  J.  Reinke  says  on  this  subject : l  'It  was 
known  from  earlier  observations  that  if  ripe  heads  of  white- 
or  yellow-grained  maize  (Zea  Mays)  were  dusted  with  pollen 
from  the  blue-  or  brown-seeded  variety,  blue  or  brown  seeds 
might  occur,  or  the  yellow  seeds  might  be  speckled  with  blue 
or  brown  spots.  Focke  gave  the  name  of  xenia  to  this  pheno- 
menon. It  became  easy  of  explanation  after  the  discovery 
of  double-fertilisation,  and  de  Vries  and  Correns  have  proved 
that  when  maize  is  dusted  with  the  pollen  of  another  variety, 
not  only  the  embryo,  but  also  the  endosperm,  shows  hybrid 

A  remarkable  contrast  to  normal  polyspermy  is  displayed 
by  the  specific  polyembryony  of  certain  parasitic  Hymenoptera. 
According  to  Silvestri,2  from  one  single  egg  of  Litomastix 
truncatellus  are  produced  about  a  thousand  sexed  and  some 
hundreds  of  sexless  larvae.  One  spermatozoon  suffices  to 
bring  about  this  extraordinary  productiveness  in  the  fertilised 
egg,  and  even  the  unfertilised  eggs,  which  need  no  spermatozoon, 
show  the  same  complicated  result  of  their  parthenogenetic 
development.  We  have  here  one  of  the  strangest  riddles  of 
life,  that  seems  to  be  in  direct  conflict  with  the  theory  of  the 
individuality  of  the  chromosomes,  but  future  generations  may 
succeed  in  solving  it. 

1  Einleitung  in  die  theoretische  Biologie,  p.  440. 

*  Un  nuovo  interessantissimo  caso  di  germinogonia  (Poliembrionia 
specified),  &c.'  (Rendiconti  delta  E.  Accademia  dei  Lincei,  Classe  d.  scienze 
fisiche,  &c.,  XIV,  1905,  pp.  534-542) ;  '  Contribuzione  alia  conoscenza  biologica 
degli  Imenotteri  Parassiti,'  I.  '  Biologia  del  Litomastix  truncatellus, ,'  Portici, 
1906  (Estr.  d.  Annali  delict  E.  Scuola  Sup.  d'Agricoltura  di  Portici,  VI).  • 




In  order  to  understand  the  importance  of  the  union  of 
germ-cells  in  the  normal  processes  of  fertilisation  in  higher 
plants  and  animals,  we  shall  do  well  to  compare  them  with 
similar  processes  in  the  lowest  forms  of  organic  life.  Let  us 
begin  with  the  conjugation  of  Infusoria. 

The  Ciliata  have  two  nuclei,  both  containing  chromatin, 
but  one — the  macronucleus — is  larger  than  the  other — the 
micronucleus.  As  Biitschli  showed,  only  the  micronucleus 
takes  an  active  part  in  conjugation,  so  that  it  may  be  called 
the  sexual  nucleus.  The  macronucleus  disappears  before 
conjugation  ;  its  activity  is  limited,  therefore,  to  the  period 
between  two  acts  of  conjugation,  when  the  ordinary  vital 
functions  are  performed,  and  it  may  be  called  the  assimilation 
nucleus,  which  controls  the  processes  of  feeding  and  movement. 

The  multiplication  of  these  tiny  Ciliata  takes  place  as  a  rule 
by  simple  division,  so  that  one  mother-cell  splits  into  two 
daughter-cells.  This  process  begins  with  indirect  division 
of  the  micronucleus,  which  forms  a  spindle  ;  it  is  only  later 
that  the  macronucleus  divides  directly  by  way  of  elongation  and 
constriction,  and  then  the  cell-body  divides.  The  micronucleus 
reveals  its  character  as  the  real  sexual  nucleus  even  at  this 
period,  but  it  does  so  more  clearly  in  the  course  of  conjugation. 

The  power  possessed  by  Infusoria  of  multiplying  by 
division  is  not  unlimited  ;  the  periods  of  division  are  interrupted 
from  time  to  time  by  the  sexual  phenomena  of  conjugation, 
by  means  of  which,  as  in  the  processes  of  fertilisation  amongst 
higher  animals,  a  reorganisation  of  the  living  substance  is 
effected.1  According  to  K.  Hertwig  and  Maupas  the  con- 
jugation of  Ciliata  (e.g.  in  Paramaecium)  takes  place  in  the 
following  way.2 

1  See    R.    Hertwig,     '  Uber   Wesen    und    Bedeutung    der    Befruchtung ' 
(SitznngsberichtederA'kad.der  Wissenschaften,Munich,  XXXII,  1902,  pp.  57-73). 

2  B.    Hertwig,    '  Uber    Befruchtung    und    Konjugation  '    ( VerhandL     der 
Deutschen  Zoolog.  Gesellsch.,  1892,  pp.  95-112) ;    also  Lehrbuch  der  Zoologie, 

1905,  p.    182  (Eng.   trans,    p.    206);    Maupas,    '  Recherches   experimentales 
sur  la  multiplication  des  Infusoires  cilies  '  (Archives  de  Zoologie  experimentale 
et  generate,  VI,  pp.    165-277) ;  see  also  Weismann,  Evolution  Theory,  Vol.  I, 
pp.   319,  &c.,  with  fig.  85  (Eng.   trans.) ;    0.    Hertwig,  Allgemeine  Biologic, 

1906,  pp.  294,  &c. 


Two  individuals  take  up  a  position  close  to  one  another, 
and  whilst  the  macronucleus  breaks  up,  the  micronucleus 
becomes  active.  In  each  individual  it  becomes  spindle- 
shaped,  and  then  divides  twice  in  succession,  so  that  each 
creature  now  possesses  four  spindles.  Of  these,  three,  which 
are  called  secondary  spindles,  gradually  degenerate,  thus 
recalling  the  polar  bodies  expelled  from  the  egg-cell.  The 
chief  or  primary  spindle  remains,  and  again  divides  into  two, 
one  of  which,  called  the  female  spindle,  remains  in  each 
individual,  whilst  the  other,  called  the  male  spindle,  passes 
into  the  adjacent  animal,  and  fuses  with  its  female  spindle. 
The  result  of  their  union  is  to  produce  in  each  animal  a  single 
new  division-spindle,  which  gives  rise  to  the  copulation-nucleus, 
and  its  development  completes  the  conjugation.  The  copula- 
tion-nucleus corresponds  to  the  cleavage-nucleus  of  the  fertilised 
ovum  ;  when  it  divides  it  forms  the  macronucleus  and  the 
micronucleus  of  the  regenerated  individual,  which  now  moves 
away  from  its  neighbour. 

We  cannot  here  discuss  in  detail  all  the  differences  between 
the  phenomena  of  conjugation  and  the  processes  of  fertilisation. 
A  comparison  of  them  shows  them  to  be  identical  in  principle. 
The  conjugation  of  two  Infusorians  aims  at  forming  in  both 
individuals  a  new  copulation-nucleus,  which  is  made  up  of  the 
chromosomes  of  the  micronucleus  of  each  in  equal  proportions. 
It  is,  therefore,  a  cross  fertilisation,  agreeing  in  its  essential 
points  with  the  processes  of  fertilisation  in  multicellular 
animals  and  plants,  and  showing  that  the  laws,  to  which  we 
have  seen  that  they  conform,  are  applicable  also  to  unicellular 
organisms.  It  may  be  mentioned  further  that  in  many 
Cryptogams  (Fucus,  Peronospora)  the  phenomena  of  conjuga- 
tion still  more  closely  resemble  the  processes  of  fertilisation 
in  higher  organisms. 

In  the  phenomena  of  conjugation  in  unicellular  animals 
and  plants,  we  can  actually  trace  the  stages  of  a  gradual 
approximation  to  the  differentiation  of  male  and  female 
germ-cells,  which  finds  its  complete  expression  in  the  fertilisa- 
tion of  higher  animals  and  plants.1  The  two  specimens  of 

1  On  this  subject  see  also  Y.  Delage,  '  Les  theories  de  la  fecondation,'  1902, 
pp.  122,  123  (Verhandl.  des  V.  internal.  Zoologenkongresses,  pp.  121-140).  The 
bearing  of  this  series  upon  the  history  of  evolution  is,  however,  as  Delage 

K  2 


Paramaecium,  whose  conjugation  has  just  been  described,  were 
exactly  similar  to  one  another  both  before  and  after  their 
conjugation.  The  same  may  be  said  of  the  daughter-indi- 
viduals, formed  by  the  subsequent  division  of  the  regenerated 
specimens  ;  each  can  in  its  turn  enter  into  conjugation  with 
another  of  its  own  kind.  There  is,  therefore,  no  difference  at 
all  in  the  sex  of  the  cells  uniting  in  conjugation.  We  might 
say  the  same  of  the  Noctiluca  miliaris,  that  causes  the  phos- 
phorescence of  the  sea,1  and  of  many  other  Infusorians.  If,  on 
the  other  hand,  we  consider  another  Infusorian,  Vorticella 
nebulifera,  we  find  a  remarkable  difference  in  the  conjugating 
individuals  ;  one  of  them,  the  macrogonidium,  is  larger  and 
represents  the  egg-cell,  whilst  the  other,  the  microgonidium, 
is  smaller,  and  represents  the  spermatozoon.  In  one  plant, 
Fucus  platy carpus,  belonging  to  a  low  Order,  we  find  a  still 
more  complete  sexual  differentiation  of  the  conjugating 
individuals  ;  round  one  relatively  enormous  spherical  egg- 
cell  swarm  numerous  diminutive  spermatozoa  destined  to 
fertilise  it. 

We  can  trace  a  distinct  advance  towards  sexual  differentia- 
tion in  the  case  of  those  Infusorians,  which  form  what  are 
called  colonies,  consisting  of  groups  of  cells,  each  being  a 
separate  individual.3 

In  Pandorina  morum  sixteen  unicellular  individuals  unite 
to  form  a  colony,  and,  at  the  time  of  sexual  reproduction, 
change  into  the  same  number  of  daughter-colonies  of  cells,  all 
resembling  one  another,  which  swarm  out  of  the  mother-colony 
and  unite  permanently  in  twos  by  way  of  conjugation.  In 
another  flagellate  Infusorian,  Eudorina  elegans,  which  also 
forms  colonies,  at  the  time  of  conjugation  two  kinds  of  daughter- 
colonies  are  produced,  distinguishable  as  male  and  female. 

rightly  remarks,  quite  hypothetical.  Cf.  also  0.  Hertwig,  pp.  304,  &c.,  where 
he  discusses  the  original  forms  of  sexual  generation  and  the  first  appearance 
of  differences  of  sex. 

1  In  Noctiluca  fertilisation  follows  conjugation  after  a  long  or  short  interval, 
and  multiplication  takes  place  by  a  budding  process  and  the  formation  of 
swarm  spores.     Cf.  0.  Hertwig,  Allgemeine  Biologic,  p.  304. 

2  The  family  of  Volvocineae,  to  which  belong  the  species  mentioned  here, 
Pandorina,  Eudorina,  and  Volvox,  enjoys  the  honour  of  being  claimed  both 
by  zoologists  and  by  botanists.     The  former  class  it  among  Flagellata,  the 
latter  among  the  Green  Algae.     Cf.  R.  Hertwig,  Lehrbuch  der  Zoologie,  1905, 
p.    171   (Eng.   trans,  pp.  ^201,  202);  Strasburger,  Lehrbuch  der  Botanik,  1904, 
p.  283  (Eng.  trans.  1908,  p.  355). 


The  female  colonies  have  sixteen  fairly  large  daughter-cells 
of  the  ordinary  shape,  and  the  male  thirty-two  much  smaller 
cells  resembling  spermatozoa  and  called  zoosperms,  whilst  the 
female  daughter- cells  are  called  oosperms.  The  zoosperms 
swarm  out  and  penetrate  the  female  daughter-colonies,  fusing 
in  conjugation  with  their  oosperms. 

A  still  higher  degree  of  differentiation  in  the  cells  and  in  the 
processes  of  conjugation  is  shown  by  the  well-known  Volvox 
globator,  which  is  also  one  of  the  Infusorians  forming  colonies. 
In  one  of  these  colonies  there  are  three  kinds  of  cells,  viz. 
somatic  or  body- cells,  which  remain  unchanged,  and  sexual 
cells  of  two  distinct  shapes,  which  are  formed  only  at  the  time 
of  conjugation.  Some  of  them  then  become  large  and  round, 
and  correspond  to  egg-cells,  whilst  others  change  into  thread- 
like zoosperms,  which  develop  in  clusters,  then  swarm  out  and 
fertilise  the  oosperms.  As  real  somatic  cells  are  developed  in 
the  Volvox  colonies,  and  serve  to  unite  the  whole  colony,  and 
perform  the  functions  of  nourishment  and  growth  for  it  as  a 
whole,  we  are  justified  in  regarding  Volvox  as  a  single  animal 
or  plant  consisting  of  body-cells  and  of  two  kinds  of  germ-cells.1 
This  is  the  link  connecting  the  unicellular  animals  (Protozoa), 
and  the  phenomena  of  their  conjugation,  with  the  multicellular 
animals  (Metazoa)  and  the  processes  of  their  fertilisation. 

In  other  Protozoa,  especially  in  the  malaria  parasites 
belonging  to  the  Haemosporidae,  the  development  of  which 
has  been  studied  chiefly  by  Grassi,3  and  in  the  allied  order 
Coccididae,  examined  at  an  earlier  date  by  Schaudinn,3  there 
are  two  periods  of  reproduction,  recurring  alternately.  The 
one  is  sexless,  but  in  the  other  there  are  present  individuals 
differentiated  in  sex,  the  so-called  macrogametes  and  micro- 
gametes,  which  unite  in  conjugation.4 

1  Cf.    also    M.    Hartmann,    '  Die    Fortpflanzungsweise    der    Organismen, 
Neubenennung  und  Einteilung  derselben,  erlautert  an  Protozoen,  Volvocineen 
und  Dicyemidcn  '  (Biolog.  Zentralblatt,  1904,  No.  1,  pp.  18-32 ;  No.  2,  pp.  33-61), 
p.  38. 

2  Cf.  Grassi's  address  at  the  Fifth  International  Zoological  Congress,  '  Da  s 
Malariaproblem  vom  Zoolog.  Standpunkt '    (Verhandl.  des   Kongresses,  1902, 
pp.  99-114). 

3  '  tiber  den!  der  Coccidien  und  die  neuere  Malaria- 
forschung  '  (Sitzungsberichte  der  Gesellsch,  naturforsch.  Freunde,  Berlin,  1899, 
No.   7,   pp.    159-78) ;     '  Der  Generationswechsel  der  Koccidien  und  Hamo- 
sporidien.  Zusammenfassende  Ubersicht '  (Zoolog.   Zentralblatt,  V,  1899,  No. 
22,  pp.  765-783). 

4  Cf.  M.  Hartmann,  as  above. 


In  the  Proceedings  of  the  German  Zoological  Society  for 
1905  (Verhandl.  der  Deutsclien  Zoolog.  Gesellschaft,  pp.  16-35 
with  Plate  I)  Fritz  Schaudinn  has  given  an  excellent  summary 
of  recent  investigations  on  fertilisation  among  Protozoa.  It 
appears  from  this  work  that  '  all  forms  of  coitus  known  to 
occur  among  other  living  organisms,  both  animals  and  plants, 
take  place  also  among  Protozoa.'  A  tabular  survey  of  these 
various  forms  of  coitus  is  given  on  pp.  20  and  21,  for  which 
Schaudinn  is  indebted  to  Stempell.1 

I  cannot  do  more  than  outline  briefly  the  processes  of  con- 
jugation in  the  lower  organisms.  They  show  an  extraordinary 
variety  of  forms,  and  are  in  many  respects  instructive  for  us 
when  we  study  the  problem  of  fertilisation.  They  teach  us 
that  the  difference  in  the  germ-cells  of  higher  animals  and 
plants  is  designed  to  render  possible  the  union  of  two  cells 
belonging  to  different  individuals,  in  order  to  effect  the  re- 
organisation of  the  vital  process  of  the  species.  The  greater 
the  difference  in  form  between  the  two  cells,  the  more  perfect 
is  their  physiological  division  of  labour  ;  inasmuch  as  the  egg- 
cell  stores  up  nourishment  for  the  development  of  the  embryo, 
and  the  sperm-cell  acquires  the  greatest  possible  agility,  in 
order  to  be  able  to  enter  the  egg-cell  and  stimulate  it  to 
development ;  and  the  more  perfectly  these  ends  are  to  be 
attained,  the  higher  is  the  degree  of  differentiation  in  the 
problem  of  fertilisation. 

The  feature  common  to  all  phenomena  of  fertilisation  is 
the  union  of  the  nuclei  of  the  two  cells,  whether  the  latter 
resemble  one  another  or  not.  We  cannot  call  the  part  taken 
by  the  centrosomes  essential  in  the  conjugation  of  lower 
animals,  for  in  most  of  them,  e.g.  in  Ciliata,  the  centrosomes 
seem  to  be  absent  or  only  temporary.  Genuine  centro- 
somes have  certainly  been  observed  in  Noctiluca,  one  of 
the  Cystoflagellata,  and  also  in  Actinosphaerium,  one  of  the 

We  may  perhaps  conclude  that  among  higher  animals  also 
the  centrosome  of  the  spermatozoon,  as  an  '  organ  of  division,' 
is  only  an  instrument  for  effecting  the  nuclear  union  of  the  two 

1  Vegetatives  Leben  und  Geschlechlsakt.      (Reprinted  from  articles  contri- 
buted by  the  Naturwissenschaftl.  Verein  in  Grief swald,  XXXVI,  1904.) 

2  Cf.  Wilson,  The  Cell,  pp.  227,  228  ;    R.  Hertwig,  Lehrbuch  der  Zoologie, 
1905,  p.  160  (Eng.  trans,  p.  190). 


germ-cells,  and  that  therefore  the  union  of  the  male  and  female 
pronuclei  is  the  essential  point  in  fertilisation,  and  through 
the  chromosomes  of  these  pronuclei  the  properties  of  both 
parents  are  transmitted  to  their  offspring. 


In  considering  the  phenomena  of  fertilisation  and  con- 
jugation (§§  3-5)  we  have  found  each  process  to  culminate 
invariably  in  the  union  of  the  nuclei  of  two  cells.  We  have 
now  to  refer  to  those  cases  in  which  there  is  no  union  of  nuclei, 
and  yet  at  least  the  beginning  of  embryonic  development 
occurs  in  the  egg  or  in  the  ovary.  A  study  of  these  cases  will 
help  us  to  arrive  at  a  general  understanding  of  the  problem  of 
fertilisation  and  heredity. 

In  the  first  place  we  must  deal  with  natural  parthenogenesis,1 
which  occurs  in  many  animals  and  plants,  and  consists  of  the 
development  of  the  egg  under  natural  conditions  without 
fertilisation  by  a  sperm-cell.  We  are  here  concerned  chiefly 
with  animal  eggs,  and  we  find  parthenogenetio  deTelopment 
occurring  especially  in  Kotatoria  among  worms,  in  Phyllopoda 
and  Ostracoda  among  Crustacea,  and  in  many  butterflies, 
(parthenogenesis  among  Psychidae  was  discovered  by  Karl 
von  Siebold  in  1848),  in  planjrlice  and  their  relations,  in 
the  praying-crickets,  gall-flies,  saw-flies,  wasps,  bees^and 
antgj.  In  considering  the  morphological  processes  during  the 
maturation  and  development  of  the  eggs  of  these  creatures,3 
we  have  to  distinguish  two  cases,  viz.  that  in  which  partheno- 
genesis takes  place  regularly  in  definite  generations,  and  is- 
obligatory  ;  and  that  in  which  it  occurs  only  incidentally,  and 
is  facultative^  It  is  true  that  in  the  first  case  parthenogenetic 

1  Under  this  heading  we  may  include  paedogenesis,  in  which  parthenogenetic 
reproduction  is  accomplished  by  animals  still  in  the  larval  stage  of  growth, 
for  instance  in  Aphididae  and  in  certain  Diptera  (Miastor  and  Chironomus). 
The  remarkable  phenomena  of  polyembryony  is  connected  with  paedogenesis  ; 
in  the  above-mentioned  Diptera,  in  one  larva  numerous  small  larvae  develop, 
and  in  the  same  way  in  some  parasitic  wasps  (in  Encyrtus  and  Polygnotus 
according  to  Marechal,  and  in  Litomastix  according  to  Silvestri)  a  number  of 
embryos  develop  in  one  egg  (see  p.   129).     Polyembryony  may  therefore  be 
described  as  a  form  of  parthenogenesis  in  the  egg  ;    especially  when  it  occurs 
in  unfertilised  eggs,  as  it  does  in  Litomastix. 

2  Cf.   Korschelt   and   Heider,   Lehrbuch  der  vergl.  Entwicklungsgesch.,  pp. 


development  is  generally,  at  least  in  animals,  not  the  exclusive 
mode  of  reproduction,  as,  at  definite  intervals  in  the  series  of 
parthenogenetic  generations,  they  are  replaced  by  sexual 
generation  (Heterogony).  The  tendency  to  parthenogenesis 
is,  however,  stronger  than  when  it  is  merely  facultative. 

A  study  of  the  maturation  of  the  eggs  of  animals  with 
obligatory  parthenogenesis  shows  that  as  a  rule  only  onejpolar 
body  is  formed,1  but  that  two  are  present  in  those  generations 
of  the  same  species  in  which  the  eggs  require  fertilisation  by 
means  of  spermatozoa.  In  these  generations  the  normal 
number  of  chromosomes  in  the  cleavage-spindle  of  the  egg  has 
subsequently  to  be  restored  by  means  of  the  male  pronucleus, 
therefore  the  number  is  first  halved  by  a  reduction  within  the 
egg,  and  made  up  again  in  the  course  of  fertilisation.2 

We  can,  therefore,  understand  why  no  reduction  takes  place, 
and  why  consequently  no  second  polar  body  is  formed,  in 
eggs  that  develop  parthenogenetically  without  fertilisation. 
That  this  is  the  case  has  been  proved  from  the  examination  of 
parthenogenetic  eggs  of  various  classes  of  animals  by  Bloch- 
mann,  Weismann,  Ishikawa,  Erlanger,  Lauterborn,  Lenssen, 
and  Woltereck.  Their  observations,  and  especially  those 
made  by  Woltereck  on  the  eggs  of  a  Crustacean  (Cypris), 
render  it  probable  that  no  reduction  in  the  number  of  chromo- 
somes takes  place  during  the  maturation  of  these  eggs,  but 
that  the  original  number  (twelve  in  Cypris)  remains  unaltered 
until  the  cleavage-spindle  is  formed,  which  constitutes  the 
first  stage  in  embryonic  development. 

According  to  0.  Hertwig,  A.  Brauer,  Viguier,  &c.,  there 
are  other  cases  in  which  a  second  polar  body  is  formed  also  in 
eggs  that  develop  parthenogenetically,  but  its  formation  is 
incomplete,  as  the  second  polar  body  remains  within  the  egg 
and  is  eventually  reunited  with  the  nucleus.  Boveri  thought 
that  the  second  polar  body  might  replace  the  spermatozoon, 
and  that  in  this  case  parthenogenesis  was  the  result  of  self- 
fertilisation  on  the  part  of  the  egg.  He  assumed  that  the 
polar  body  served,  instead  of  the  sperm-nucleus,  to  restore  the 
normal  number  of  chromosomes  for  the  first  cleavage-spindle 

1  This  has  been  confirmed  recently  by  J.  P.  Stschelkanovzew's  examination 
of  plant-lice.     Cf.  his  article  '  Uber  die  Eireifung  bei  viviparen  Aphiden  ' 
(Biolog.  Zentralblatt,  1904,  No.  3,  pp.  104-112). 

2  Cf.  pp.  110  and  120. 


of  the  egg.  According  to  Brauer  there  are  two  types  of 
development  in  the  parthenogenetic  eggs  of  Artemia.  In 
one  type  the  second  polar  body  is  formed,  but  united  again 
with  the  egg-nucleus  ;  in  the  other  type  it  is  not  formed  at  all. 
Brauer  states  that  in  the  first  type  the  number  of  chromosomes 
in  the  cleavage-spindle  of  the  egg  is  168  (the  normal  number 
for  this  species)  ;  in  the  second  type  it  is  only  84  (half  the 
normal  number),  but,  as  no  division  takes  place,  the  chromo- 
somes have  a  double  value. 

The  maturation  of  the  egg  of  the  parasitic  Litomastix 
truncatellus,  as  observed  by  Silvestri  in  1905,  is  remarkably 
interesting  (see  p.  129,  note  2).  The  process  is  the  same  in 
the  parthenogenetic  as  in  the  fertilised  egg.  In  both  cases 
two  polar  bodies  (globuli  polari)  are  formed,  and  remain  in  the 
front  part  of  the  egg.  The  first  polar  body  divides,  but  its 
two  halves  unite  with  one  another  and  with  the  second  polar 
body  to  form  a  nucleus,  which  Silvestri  calls  from  its  origin 
a  polar  nucleus. 

In  many  insects  however,  especially  in  such  as  have  only 
facultative  parthenogenesis,  e.g.  in  Liparis,  Bombyx  and 
Leucoma  among  butterflies,  and  in  the  honey-bees  and  many 
ants  (Lasius)  among  Hymenoptera,  the  maturation- divisions 
of  the  parthenogenetic  egg  result  in  the  complete  formation 
and  separation  of  two  polar  bodies.  At  Weismann's  suggestion, 
Dr.  Petrunkewitsch  *  made  a  very  careful  examination  of 
the  unfertilised  eggs  of  the  bee,  from  which  drones  are  hatched, 
and  showed  this  quite  conclusively.  We  can,  perhaps,  account 
for  the  formation  of  two  polar  bodies  by  assuming  that,  in 
these  insects,  fertilisation  is  the  normal  condition  ;  where  it 
does  not  take  place,  the  egg  makes  the  same  preparations  for 
it  as  when  it  does.  But  in  many  gall-flies  (Rhodites)  partheno- 
genesis  is  obligatory,  and  yet  two  fully  developed  polar  bodies 
are  formed,  neither  of  which  reunites  with  the  egg.  It  is  a 
remarkable  fact  that  when  two  such  polar  bodies  have  been 
cast  out  of  the  egg,  and  when  the  accompanying  karyokineses 
have  reduced  the  number  of  chromosomes  in  the  egg  by  a  half, 
the  normal  number  nevertheless  recurs  in  the  cleavage-spindle. 

1  *  Die  Richtungskorper  und  ihr  Scbicksal  im  befruchteten  und  unbefruch- 
teten  Bienenei '  (Zoolog.  Jahrbiicher,  Abteilung  fur  Anatomie  u.  Ontogenie, 
XIV,  1901). 


Petrunkewitsch  observed  this  phenomenon  in  the  eggs  of  the 
bee,  but  was  unable  to  account  for  it. 

Morphological  processes  closely  resembling  parthenogenesis 
in  the  animal  kingdom  occur  in  the  parthenogenetical  develop- 
ment _of  many  plants.     In  1900  Juel  observed1  that  in  Anten- 
j       naria  alpina  the  egg  developing  parthenogenetically  in  the 
/       embryo-sac  shows  no  reduction  in  the  number  of  chromosomes  ; 
and  in  1901  the  same  thing  was  observed  by  Murbeck3  in 
the  varieties  of  Alchimilla  that  develop  parthenogenetically. 
\      In  1905  E.  Strasburger  devoted  much  attention  to  the  study 
of  the  propagation  of  the  Eualchimillae,  and  came  to  the 
conclusion  that  in  the  seeds  of  these  plants  the  development 
of  the  mother-cell  of  the  embryo-sac  and  of  the  embryo  takes 
V  place  without  fertilisation.     In  this  case  there  is  no  reduction 
\  in  the  original  number  of  chromosomes,  which  remains  constant 
as  in  the  somatic  cells  of  the  plant.       Strasburger  prefers  to 
call    this    process    apogamyj    or    sexless    propagation,    rather 
than   parthenogenesis,  or  unisexual    propagation,  because  it 
takes  place  on  vegetative  and  not  sexual  lines.     Winkler,  on 
the    other    hand,    retains    the  name   '  parthenogenesis,'    but 
calls  it  in  this  case  somatic,  as  opposed  to  the  true  generative 

In  one  of  the  Algae  (Ectocarpus  siliculosus)  an  extraordinary 
phenomenon  has  been  observed.  Not  only  the  female  germ- 
cell  can  develop  parthenogenetically  under  certain  circum- 
stances, but  the  male  cell  may  also  do  so  ;4  in  this  case,  however, 
the  difference  in  size  between  the  two  is  not  great,  and  the 
male  plant,  corresponding  with  the'  smaller  size  of  the  zoo- 
sperm,  tends  to  be  poorly  developed.  This  is  the  only  case, 
occurring  under  natural  conditions,  of  male  parthenogenesis 
or  arrhenogenesis. 

There  are  many  obscure  points  in  natural  parthenogenesis, 
as  we  have  shown.  Only  one  fact  can  be  stated  with  certainty, 

1  '  Vergleichende  Untersuchungen  iiber  typische  und  parthenogenetische 
Fortpflanzung  bei  der  Gattung  Antennaria  '  (Svenska  Vetenskaps  Akad.  Handl, 
XXXIII,  1900,  n.  5). 

2  *  Parthenogenetische  Embryobildung  in  der  Gattung  Alchimilla  '  (Lunds 
Univers.  Arsskri/t,  XXXVI,  1901,  n.  2).  ' 

3  Cf.    Strasburger,     '  Die    Apogamie    der    Eualchimillen    und    allgemeine 
Gesichtspunkte,    die    sich    daraus    ergeben '    (Jahrbiicher    fur     wissenschaftL 
Botanik,  LXI,  1905,  pp.  88-164).     Cf.  also  the  article  in  the  Naturwissenschaft- 
liche  Rundschau,  XX,  1905,  No.  27,  pp.  342-344. 

4  Weismann,  Lectures  on  the  Evolution  Theory,  I,  334. 


viz.  that,  in  a  good  many  kinds  of  animals  and  plants,  the 
egg-nucleus  alone  is  able  to  begin  the  embryonic  development 
of  the  egg.  Therefore  the  union  of  the  nuclei  of  two  cells, 
the  male  and  female  germ-cells,  is  not  absolutely  and  universally 
essential  to  the  beginning  of  embryonic  development,  even  in 
those  organisms  which  possess  both  kinds  of  germ-cells.  If 
nevertheless,  in  normal  fertilisation,  the  union  of  the  nuclei 
of  the  two  germ-cells  is  regularly  the  culminating  point  of  the 
whole  process,  its  object  is  not  merely  to  stimulate  the  ovum 
to  embryonic  development,  but,  over  and  above  this,  its  object 
is  chiefly  to  secure  the  benefits  of  amphimixis,  i.e.  the  trans- 
mission of  the  combined  properties  of  both  parents  to  their 
offspring,  and  this  is  brought  about  by  the  union  of  the  paternal 
and  maternal  nuclear  elements  in  the  cleavage-spindle  of  the 
fertilised  ovum..  We  must  not,  however,  undervalue  the  first 
object  in  normal  fertilisation.  It  cannot  be  denied  that  a 
renewal  of  the  capability  of  development  of  the  species,  a 
*  reorganisation  of  the  living  substance,'  is  connected  with  the 
union  of  the  germ-cells,  and  therefore  it  is  still  very  doubtful 
whether  an  unlimited  propagation  by  parthenogenesis  would 
be  possible,  at  least  in  the  animal  kingdom.1 


Let  us  now  turn  to  experiments  in  artificial  parthenogenesis.2 
Tichomirow  discovered  in  1886  3  that  in  the  eggs_of  the_silk- 
moth,  which  otherwise  require  fertilisation,  parthenogenesis 
may  be  produced  by  rubbing  them  between  cloths.  The 
same  result  was  obtained  by  Tichomirow  both  in  1886  and  in 
1902  by  dipping  the  eggs  into  concentratedsulphuric  and 

1  In  one  Crustacean  (Cypris  reptans)  Weismann  states  that  he  observed 
uninterrupted  parthenogenesis  (Zoolog.  Anzeiger,  XXVIII,  1904,  p.  39).     It 
seems  to  be  possible  also  in  some  grasshoppers  which  are  all  females  (de  Sinety, 
Recherches  sur  les  phasmes,  1901,  pp.  13,  &c.).     H.  Schmitz  has  made  the  same 
observation   in   Dixippus    morosus,    a    tropical     praying- cricket    ('  Dixippus 
morosus,'  in  Natur  und  Offenbarung,  1906,  Part  7,  pp.  385-407,  402,  &c.). 

2  A  summary  of  these  experiments  is  given  by  Korschelt  and  Heider, 
Lehrbuch  der  vergl.  Entwicklungsgesch.,  pp.  623,  &c.,  663  &c. ;  by  Boveri,  Das 
Problem  der  Befruchtung,  pp.  39,  &c. ;  by  Y.  Delage,  Les  theories  de  la  fcconda- 
tion,  pp.  135,  &c. ;  by  Kathariner,  Das  Problem  der  Befruchtung,  pp.  518,  &c. ; 
by  0.  Hertwig,  Allgemeine  Biologie,  pp.  326,  &c. 

3  '  Die  kiinstliehe    Parthenogenese  bei  Insekten '    (Archiv  f.   Anatomie   u. 
Physiologic,  Supplement,  1886). 


muriatic  acid.  In  1887  0.  and  K.  Hertwig  l  found  that  un- 
fertilised eggs  of  sea-urchins  could  develop  under  the  influence 
of  external  stimulus,  and  E.  Hertwig  continued  these  experi- 
ments in  1888  and  1896,  and  in  a  work  2  published  in  the  latter 
year  he  describes  the  processes  of  division  in  the  egg-nucleus 
which  result  from  placing  the  unfertilised  egg  of  a  sea-urchin 
in  a  weak  solution  of  strychnine.  Many  experiments  in 
artificial  parthenogenesis  have  been  made  in  the  last  few  years 
by  American  naturalists,  Th.  Morgan,  Jacques  Loeb,  E.  B. 
Wilson,  and  A.  B.  Mathews,  and  also  by  scientists  of  other 
nationalities,  such  as  Y.  Delage,  Giard,  Bataillon,  Henneguy, 
Herbst,  Winkler,  Prowazek,  Kostanecki,  Boveri,  WasiliefT. 
Schiicking,  Petrunkewitsch,  &c.3 

Unfertilised  eggs  of  very  various  animals  (Echinoderms, 
Medusae,  Molluscs,  Annelids,  insects  and  fishes)  were  chosen 
and  exposed  to  chemical,  physical,  and  mechanical  stimuli  of 
many  different  kinds.  Solutions  of  various  poisons,  narcotics 
and  salts,  such  as  strychnine,  nicotine,  hyoscyamine,  ether, 
alcohol,  chloroform,  calcium  chloride,  magnesium  chloride, 
diphtheria  serum,  a  solution  of  cane  sugar,  urea,  and  sperm 
extract — all  proved  efficacious  in  setting  up  the  processes  of 
development ;  and  similar  results  were  obtained  by  concen- 
trating the  sea-water  containing  the  eggs,  by  dipping  them  in 
warm  sea-water  and  by  applying  galvanic  currents  and 
mechanical  vibration.  Jacques  Loeb's  experiments  were  the 
most  successful.  He  was  able  to  cause  the  unfertilised  eggs 
of  all  kinds  of  Echinoderms  and  Annelids  to  form  larvae,  and 
by  subjecting  those  of  sea-urchins  to  the  action  of  chloride  of 
magnesium  for  two  or  three  hours  he  made  them  develop  as 

1  '  Uber  den  Befruchtungs-  und  Teilungsvorgang  des  tierischen  Eis  unter 
dem  Einflusse  ausserer  Agentien  '  (Jenaische  Zeitschr.  fur  Naturwissenschaft, 

2  Uber  die  Entwicklung  des  unbefruchteten  Seeigeleis,   Festschrift  fur   C. 
Gegenbaur,  Leipzig,  1896. 

3  Korschelt  and  Heider  give  a  list  of  books  dealing  with  the  subject,  pp.  733, 
&c.     They  do  not,  however,  mention  those  of  the  last  four  authors  named 
above  :  Boveri, '  Zellenstudien,'  1902,  Part  4,  p.  9  ;  Wasilieff,  '  Uber  kunstliche 
Parthenogenesis  des  Seeigeleis  '    (Biolog.  Zentralblatt,  XXII,   1902,  No.   24, 
pp.  758-772) ;  A.  Schiicking, '  Zur  Physiologic  der  Befruchtung,  Parthenogenese 
und    Entwicklung '     (Archiv  fur  die  ges.   Physiologie,    XCVII,     1903) ;    A. 
Petrunkewitsch,  '  Kunstliche  Parthenogenese  '  (Zoolog.  Jahrbiicher,  Supplem. 
VII,  1904,  '  Festschrift  flir  Weismann,'  pp.  77-138).     Cf.  also  a  review  of  the 
last-mentioned  article  in  the  Naturwissenschaftliche  Rundschau,  1904,  No.  35, 
pp.  444,  &c. 


far  as  the  blastula  stage,  and  finally  even  as  far  as 
that  of  the  Pluteus  larva.  These  larvae  remained  alive 
for  as  long  as  ten  days,  but  were  unable  to  form  any 
calcareous  skeleton,  although  they  developed  this  power  when 
carbonate  of  calcium  was  added  to  the  sea-water.  Loeb 
succeeded  also  in  inducing  the  eggs  of  an  Annelid  (diaeto- 
pterus)  actually  to  reach  the  stage  of  forming  the  trocho- 
phore  larva.1  These  careful  and  ingenious  experiments  seem 
to  have  resulted  in  the  discovery  of  a  magic  wand,  capable 
of  awakening  the  life  dormant  in  the  unfertilised  animal 
ovum ;  and  apparently  they  afford  a  brilliant  confirmation  of 
Aristotle's  opinion,  for  he  believed  the  ovum  to  contain  the 
essentials  of  each  animal  species,  and  the  spermatozoon  merely 
to  have  the  effect  of  stimulating  the  ovum  to  develop.  Before 
we  assent  to  these  conclusions,  we  must  examine  the  results 
of  these  experiments  somewhat  more  closely. 

The  forms  resulting  from  artificially  produced  partheno- 
genesis differ  in  many  respects  from  the  normal,  as  Kathariner 
already  partially  pointed  out  in  '  Natur  und  Offenbarung,' 
1903,  p.  518.  Their  cleavage-globules  have  less  power  of 
resistance  ;  they  show  a  tendency  to  fall  to  pieces,  and  dwarf 
larvae  develop  from  the  fragments,  or  else  several  cleavage- 
globules  unite  and  give  rise  to  gigantic  and  monstrous  embryos. 
In  the  sea-urchin  larvae  produced  parthenogenetically,  irregu- 
larities in  the  formation  of  the  skeleton  are  apt  to  occur, 
and  all  these  artificially  developed  forms  seem  to  lack  some 
directive  power,  which  is  supplied  by  normal  fertilisation 
and  results  in  development  on  definite  lines.  The  Pluteus 
and  trochophore  larvae,  produced  by  Loeb's  experiments,  are 
the  highest  achievements  of  artificial  parthenogenesis,  but  it 
is  doubtful  whether  they  were  really  capable  of  continued 
existence  and  of  developing  from  the  stage  of  larvae  to  that 
of  adults,  for  hitherto  no  one  has  succeeded  in  breeding  even 
the  natural  larvae  of  these  species  in  a  laboratory.  In  any 

1  Loeb,  '  On  the  nature  of  the  process  of  fertilisation  and  the  artificial 
production  of  normal  larvae  (Plutei)  from  the  unfertilised  eggs  of  the  sea- 
urchin  '  (American  Journal  of  Physiology,  III,  1899) ;  '  On  the  artificial  pro- 
duction of  normal  larvae  from  the  unfertilised  eggs  of  the  sea-urchin  '  (1900) ; 
'  Further  experiments  on  artificial  parthenogenesis  '  (IV,  1900) ;  '  Experiments 
on  artificial  parthenogenesis  in  Annelids  (Chaetopterus)  and  the  nature  of  the 
process  of  fertilisation  '  (IV,  1901). 


case,  although  in  a  few  successful  instances  larvae  were  actually 
formed,  there  were  many  less  successful,  or  even  quite  un- 
successful, attempts  at  artificial  parthenogenesis,  in  which  the 
cleavage  process,  artificially  induced,  ceased  even  earlier. 

An  attempt  to  account  for  these  variations  has  been  made 
by  Boveri  ('  Das  Problem  der  Befruchtung,'  pp.  39,  &c.)  in  his 
criticism  of  Morgan  and  Wilson's  experiments.  He  points  out 
that,  when  an  ovum  is  fertilised,  only  one  radiation  sphere  is 
formed  at  the  head  of  the  spermatozoon  that  has  entered. 
The  division  of  this  radiation  sphere  gives  rise  to  the  two 
astrospheres  which  are  the  poles  of  the  first  nuclear  spindle  of 
the  ovum.  (Cf.  p.  122  and  Plate  I,  figs.  1-7.)  According 
to  the  observations  of  the  two  American  writers,  however, 
artificial  parthenogenesis  of  the  same  eggs,  under  the  influence 
of  Loeb's  reagents,  results  in  the  formation  of  a  fluctuating, 
but  often  considerable,  number  of  radiation-spheres,  each  of 
which  has  a  newly  formed  centrosome  as  its  centre.  Boveri 
believes  that  regular  cleavage  of  the  ovum  can  occur  only  in 
the  exceptional  case  that  only  two  really  active  radiation- 
spheres  develop  and  take  up  their  positions  at  opposite  poles 
of  the  egg-nucleus  ;  under  all  other  circumstances  the  numer- 
ous division-centres,  having  no  orderly  arrangement,  act  as 
they  do  in  pathological  polyspermy,  and  give  rise  to  an  irregular 
mass  of  cells,  which  speedily  dies.  Therefore  Boveri  still 
regards  the  introduction  of  the  spermatozoon  into  the  ovum 
as  supplying  the  directive  quality,  which,  in  normal  fertilisa- 
tion, secures  the  formation  of  a  regular  cleavage-spindle  with 
two  poles.  It  is  comparatively  of  less  importance  whether 
the  spermatozoon  actually  brings  its  own  centrosome  with 
it  into  the  ovum,  or  whether,  through  the  chemical  and 
physical  action  of  the  sperm-nucleus,  the  egg-protoplasm 
becomes  capable  of  forming  a  new  centrosome  for  itself, 
which  then  takes  up  a  position  in  front  of  the  sperm-nucleus, 
and  by  dividing  forms  the  poles  of  the  cleavage-spindle.  The 
attempts  at  artificial  parthenogenesis  seem  to  me  to  support 
the  theory  of  the  new  formation  of  centrosomes  in  the  ovum  ; 
and  these  experiments  have  in  some  degree  caused  me  to 
modify  the  account  that  I  previously  gave  (see  p.  125)  of  the 
significance  of  the  normal  process  of  fertilisation,  in  giving 
which  I  was  guided  by  Boveri's  diagrams.  (Plate  I,  figs.  1-7.) 


Another  remark  must  be  made  on  the  subject.  Morgan,1 
and  still  more  emphatically  Wilson,3  declare  that  they  have 
observed  the  new  formation  of  centrosomes  as  centres  of  the 
radiation  spheres  in  sea-urchins'  eggs  parthenogenetically 
developed  by  the  application  of  chloride  of  magnesium,  and 
Wilson  describes  the  new  formation  of  centrosomes  in  non- 
nucleated  fragments  of  an  egg.3  Wasilieff,  on  the  other  hand,4 
in  his  corresponding  experiments  with  strychnine,  nicotine 
and  hyoscyamine,  found  that  the  first  divisions  took  place 
without  the  formation  of  centrosomes,  which,  if  they  appeared 
at  all,  did  so  only  in  the  later  stages  of  cleavage,  and  were  then 
formed  of  the  nuclear  substance  of  the  cells.  The  occurrence 
of  true  centrosomes  in  non-nucleated  fragments  of  an  egg  is 
questioned  also  by  Petrunkewitsch.5 

Should  the  observations  of  Wasilieff  and  Petrunkewitsch 
find  confirmation,  we  shall  have  greater  reason  for  regarding 
the  centrosomes,  not  as  a  permanent  formation,  but  as  only 
a  temporary  biomechanical  means  of  assisting  the  process  of 
cell-division.  (Cf .  Chapter  V,  pp.  98,  &c.)  If  this  be  so,  we  must 
consider  the  appearance  of  a  centrosome  beside  the  sperm- 
nucleus  in  normal  fertilisation  of  the  animal  ovum,  not  as  the 
cause  of  cell- division,  but  as  a  consequence  of  the  beginning  of  the 
process.  We  shall  then  have  to  agree  with  Oskar  Hertwig's 
older  theory  of  nuclear  fertilisation,  and  say,  that  in  normal 
fertilisation  also,  the  entrance  of  the  sperm-nucleus  into  the 
ovum  and  its  union  with  the  female  pronucleus  constitute  the 
real  elements  of  fertilisation. 

The  question  of  chromatin-reduction  is  another  point 
connected  with  artificial  parthenogenesis  on  which  opinions 
are  divided.  The  eggs  used  in  the  experiments,  to  which  I 
have  referred,  were  such  as  had  undergone  their  maturation- 
divisions,  and  so  we  must  assume  that  the  nucleus  of  each 
contained  only  half  the  number  of  chromosomes  peculiar  to  the 

1  '  The  production  of  artificial  astrospheres  '   (Archiv  fur  Entwicklungs- 
mechanik,  III,  1896). 

2  '  Experimental    studies  in    cytology,'   I.   '  Artificial  parthenogenesis  in 
sea-urchin  eggs  '  (Ibid.  XII,  1901). 

3  '  Cytasteren  und  Centrosomen  bei  kiinstlicher  Parthenogenese  '  (Zoolog. 
Anzieger,  XXVI,  1904,  pp.  8-12). 

4  '  tiber    kiinstliche  Parthenogenesis  des  Seeigeleis  '  (Biolog.  Zentralblatt, 
1902,  pp.  758-772). 

5  '  Kiinstlicho  Parthenogenese  '  (Zoolog.  Jahrbiicher,  Supplem.,  VII,  1904, 


species.  Wilson  states  expressly  that  he  found  eighteen  and 
not  thirty-six  chromosomes  in  the  cleavage-cells  of  the  sea- 
urchins'  eggs  undergoing  pa-rthenogenetic  development.  Y. 
Delage,  however  says  that  in  his  experiments  on  the  same 
eggs  he  found  the  normal  number  of  chromosomes  to  be 
restored.  Boveri  argues1  that  eighteen,  which  Delage  appar- 
ently took  to  be  the  normal  number,  is  really  the  reduced 
number  for  that  species,  for  his  own  observations  and  those 
of  E.  Hertwig  both  show  thirty-six  to  be  the  normal.  We 
must  probably  assume  that,  when  eggs  develop  by  artificial 
parthenogenesis,  half  the  normal  number  of  chromosomes 
suffices  for  the  cleavage-nucleus  of  the  developing  ovum. 
Petrunkewitsch  has  gone  so  far  as  to  state  (1904)  one  essential 
difference  between  artificial  and  natural  parthenogenesis  to  be 
that,  in  the  former,  the  reduced  number  of  chromosomes 
invariably  remains. 

We  may  now  turn  to  the  more  general  conclusions  formed 
by  various  students,  as  resulting  from  the  experiments  in 
artificial  parthenogenesis. 

Loeb  thinks  he  is  justified  by  his  experiments  (see  p.  140) 
in  concluding  that  the  ova  of  many,  and  perhaps  of  all,  animals 
have  a  certain  tendency  to  develop  parthenogeneticaliy,  but 
as  a  rule  this  development  is  so  slow  that  the  ovum  perishes 
before  it  attains  to  any  advanced  stage  of  cleavage.  Artificial 
stimuli,  such  as  salt  solutions,  &c.;  by  hastening  the  develop- 
ment, enable  the  ovum  to  attain  its  end  parthenogenetically. 
Korschelt  and  Heider,  on  the  contrary,2  and  E,  Hertwig3 
incline  to  the  far  more  moderate  opinion  that  the  chemical 
and  physical  stimuli  are  able  to  set  up  in  the  mature,  but  still 
unfertilised,  ovum  that  reciprocal  action  of  the  parts  (and 
especially  of  the  cytoplasm  and  nucleus)  which  is  indispens- 
able to  embryonic  development,  and  which  under  normal  con- 
ditions results  only  from  fertilisation.  Boveri4  thinks  that  the 

1  '  "Dber  mehrpolige  Mitosen  als  Mittel  zur  Analyse  des  Zellkerns '  ( Verhandl. 
der  physikal.-mediz.  Gesellsch.,  Wiirzburg,  XXXV,  1902). 

2  Lehrbuch  der  vergl.  Entwicklungsgesch.,  p.  624 ;  cf.  also  ibid.  pp.  65-67. 

3  '  tJber  Korrelation  von  Zell-  und  Kerngrosse  und  ihre  Bedeutung  fiir 
die  geschlechtliche  Differenzierung  und  die  Teilung  der  Zelle  '  (Biolog.  Zentral- 
blatt,   1903,  Nos.   2  and  3) ;    also    *  Uber  das    Wechselverhaltnis  von  Kern 
und  Protoplasma,'  Munich,  1903.     (Reprinted  from  the  Miinchener  Medizin, 
Wochenschrift,  I.) 

4  Das  Problem  der  Befruchtung,  pp.  22-23,  39,  &c. 


phenomena  observed  in  artificial  parthenogenesis  afford  a  con- 
firmation of  his  theory  of  fertilisation,  according  to  which  the 
mature  ovum  resembles  a  complete  piece  of  mechanism,  still 
at  rest,  and  needing  only  to  be  wound  up,  in  order  to  begin  to 
work.  The  key  to  it  is,  in  normal  fertilisation,  the  centrosome 
of  the  spermatozoon ;  but  in  artificial  parthenogenesis  it 
consists  of  some  chemical  or  physical  agents  ;  which  affect  the 
egg-plasm  in  a  way  similar  to  the  action  of  the  centrosome 
under  ordinary  circumstances.  As  early  as  1886  Tichomirow 
put  forward  the  theory  that  the  egg-cell  responded  to  all 
exterior  action — no  matter  of  what  kind — invariably  in  the 
same  way,  peculiar  to  itself,  viz.  by  development ;  just  as 
the  cells  of  the  optic  nerves  react  invariably  through  their 
susceptibility  to  light,  and  the  cells  of  the  muscular  fibres 
contract  under  external  stimulus.  This  idea  was  borrowed 
from  Johannes  Miiller's  law  of  specific  energies  of  the  senses. 
The  same  view  has  been  recently  formulated  by  Y.  Delage  in 
the  following  terms :  ]  '  The  mature  but  still  unfertilised 
ovum  is  in  a  condition  of  unstable  equilibrium  ;  any  stimulus, 
destroying  the  equilibrium,  gives  rise  to  development.' 

Loeb  goes  perhaps  rather  too  far  when  he  says  that  all 
animal  ova  have  an  original  tendency  to  parthenogenetic 
development,  for  the  results  of  experiments  show  that  artificial 
parthenogenesis  seldom  attains  the  normal  end,  and  that  the 
cleavage  stages  thus  produced  cease,  as  a  rule,  without  develop- 
ing to  a  larva.  Moreover,  at  the  present  time  most  zoologists 
agree  in  regarding  natural  parthenogenesis,  where  it  actually 
occurs  among  animals,  not  as  the  original  mode  of  develop- 
ment, but  as  a  later  simplification  of  the  original  mode ; 
they  believe  propagation  by  fertilisation  to  be  the. normal 

We  must  therefore  not  overestimate  the  capacity  of  many 
eggs  to  develop  without  fertilisation  under  artificial  stimulus  ; 
but,  on  the  other  hand,  we  must  not  underestimate  it,  for,  taken 
in  conjunction  with  natural  parthenogenesis,  it  proves  plainly 
enough  that  under  certain  circumstances  one  nucleus  alone, 
viz.  the  egg-nucleus,  suffices  to  begin  embryonic  development. 
The  chief  object,  then,  of  the  union  of  two  different  nuclei  in 
normal  fertilisation  is  not  merely  to  stimulate  the  ovum 

1  Les  theories  de  la  fecondation,  p.  138. 


to  develop,  but  rather  to  secure  the  benefits  of  amphimixis,  i.e. 
of  transmitting  to  the  offspring  the  properties  of  both  parents, 
and  this  is  effected  by  the  union,  in  the  cleavage-spindle  of 
the  ovum,  of  the  nuclear  elements  of  the  male  and  female 
pronuclei.  I  shall  recur  to  this  subject  at  the  end  of  the 
present  chapter. 

The  other  object  of  fertilisation,  viz.  to  stimulate  the 
ovum  to  develop,  can  be  attained  by  very  various  means 
without  fertilisation,  as  the  experiments  in  artificial  par- 
thenogenesis prove.1 

As  Delage  puts  it  the  mature  egg  really  gives  us  the  im- 
pression of  being  in  a  state  of  unstable  equilibrium  ;  anything 
that  disturbs  that  equilibrium  suffices  to  cause  the  egg  to 

Closely  akin  to  this  idea  is  the  further  suggestion  that, 
in  normal  fertilisation  also,  there  may  be  certain  chemico- 
physical  processes  which  result  in  the  development  of  the 
egg.  Thus  we  arrive  at  the  physical  and  chemical  theories 
of  fertilisation,  which  have  been  propounded  in  the  last  few 
years.  They  are  still  hardly  ripe  for  discussion,  and  consist 
chiefly  of  rather  vague  speculations,  so  we  may  limit  ourselves 
to  what  is  absolutely  necessary  in  dealing  with  them. 

The  question  to  be  answered  is  :  'In  normal  fertilisation, 
what  does  the  spermatozoon  bring  into  the  ovum  to  render 
it  capable  of  development  ?  '  The  answer  given  by  Boveri's 
morphological  theory  is  :  'In  its  centrosome  the  spermatozoon 
imports  a  new  division-centre  into  the  ovum.'  The  physical 
and  chemical  theories,  however,  reply :  '  The  spermatozoon 
produces  in  the  ovum  certain  physical  and  chemical  changes 
which  result  in  the  process  of  division.' 

The  two  classes  of  theories  are  not  necessarily  antagonistic, 
but  are  complementary.  Carnoy  and  Biitschli  had  already 
suggested  that  the  centrosomes  stimulate  the  cell  to  divide, 
by  exerting  some  chemical  influence  on  the  protoplasm, 
and  Boveri  himself  expressed  an  idea,  which  Wilson  subse- 
quently elaborated,  that  possibly  some  chemical  substance, 

1  I  must  remind  the  reader  here,  as  I  did  on  p.  141,  that  this  object  is  only 
imperfectly  attained  by  artificial  parthenogenesis.  We  must  therefore 
assume  that  a  particular  kind  of  '  reorganisation  of  the  vital  substance  ' 
is  connected  with  natural  fertilisation,  and  especially  with  the  union  of  the 


stimulating  the  ovum  to  develop,  is  brought  into  it  by  the 

The  morphological  theory  only  shows  itself  really  anta- 
gonistic to  the  chemico-physical  theory,  when  there  is  a  choice 
between  one  or  other  of  them,  as  being  exclusively  valid  ;  J. 
Loeb  seems  inclined  to  adopt  the  chemico-physical  theory, 
in  spite  of  the  obscurity  in  which  it  is  still  involved.  There 
is  a  wide  divergency  of  opinions  regarding  the  nature  of  the 
chemical  and  physical  processes  which  underlie  fertilisation. 
Loeb,  the  chief  champion  of  the  new  theory,  originally  thought 
that  electrolysis  might  account  for  it,  and  that  new  metallic 
ions  were  brought  by  the  spermatozoon  into  the  ovum.  Subse- 
quently, he  came  to  the  conclusion  that  some  alteration  in 
the  osmotic  conditions  of  the  ovum  was  effected  by  the  action 
of  the  spermatozoon.  In  1900,  Wilson  suggested  that  the 
middle-piece  of  the  spermatozoon,  which  contains  its  centro- 
some,  might  be  the  bearer  of  a  specific  chemical  substance 
stimulating  the  ovum  to  development,  quite  apart  from  the 
sperm-nucleus.  Finally,  Yves_Delage  has  set  up  a  still  simpler 
hypothesis  of  chemical  and  physical  fertilisation ;  e  he  thinks 
that  the  ovum  becomes  capable  of  fertilisation  in  consequence 
of  the  breaking  up  of  the  nuclear  membrane  during  the  matura- 
tion-divisions, and  the  distribution  of  the  nuclear  fluid  to  the 
protoplasm  of  the  ovum.  The  head  of  the  spermatozoon 
penetrating  the  ovum  becomes  the  male  pronucleus,  and 
grows  by  taking  up  water  from  the  egg-plasm,  thus  depriving 
it  of  some  of  its  fluid.  In  this  dehydration  of  the  ovum  by 
the  sperm-nucleus  Delage  thinks  he  has  discovered  the  chemico- 
physical  cause  of  the  beginning  of  the  dividing  process  in  the 
ovum.  He  does  not,  however,  exclude  the  specific  action  of 
salts,  metallic  ions,  &c.,  which  may  be  contained  in  the  sperm- 

Loeb  considered  that  his  experiments  in  artificial  partheno- 
genesis had  transferred  the  problem  of  fertilisation  from  the 
domain  of  morphology  into  that  of  chemico-physical  science. 

1  .Cf.  Korschelt  and  Heider,  Lehrbuch  der  vergl.  Entwicklungsgesch.,  pp.  663, 
&c.,  and  Wilson,  The  Cell,  pp.  354,  &c.     The  phenomena  of  natural  partheno- 
genesis are  against  these  theories,  as  in  that  case  there  is  no  spermatozoon, 
nor  any  special  chemical  stimulus,  present. 

2  On  this  theory  and  those  akin  to  it,  see  Y.  Delage,  Les  theories  de  la 
fecondation,  pp.  135,  &c. 

L  2 


Y.  Delage  seems  to  share  this  opinion,  and  Max  Verworn  has 
long  desired  to  replace  the  morphological  theory  of  fertilisation 
by  a  physiological  one.  Quite  recently  B.  Hatschek  too  has 
brought  forward  a  new  '  Hypothesis  of  organic  inheritance  ' 
('  Hypothese  der  organischen  Vererbung,'  Leipzig,  1905)  based 
upon  a  physiological  and  chemical  foundation.  I  agree  with 
Boveri l  in  thinking  that  this  bold  speculation  is  still  far  from 
having  a  basis  of  ascertained  scientific  facts.  After  showing 
what  a  vast  number  of  distinct  morphological  problems  are 
involved  in  the  fertilisation,  cleavage,  and  embryonic  develop- 
ment of  the  ovum,  with  regard  to  the  physical  and  chemical 
factors  of  which  we  still  know  nothing  at  all,  Boveri  continues  : 
'  As  we  have  said,  a  transference  of  the  problem  of  fertilisation 
into  the  domain  of  physico-chemical  science  would  involve 
the  assumption  that  the  process  of  cell-division  has  been  traced 
back  to  physical  and  chemical  factors.  How  far  we  really 
are  from  having  accomplished  this  is  known  to  everyone  who 
has  studied  the  question  ;  and  it  is  scarcely  possible  at  the 
present  time  to  speculate  as  to  how  deeply  we  may  eventually 
penetrate  into  the  mystery.' 

The  problem  of  fertilisation  and  heredity  is,  at  any  rate,  no 
merely  morphological  problem  ;  on  the  contrary,  its  physio- 
logical aspect  is  the  chief  point,  as  enabling  us  to  understand 
the  morphological  processes,  but  the  morphological  and 
physiological  aspects  must  be  taken  in  conjunction  to  support 
and  complete  one  another. 

My  opinion  regarding  the  importance  of  artificial  partheno- 
genesis as  bearing  upon  the  problem  of  fertilisation  may  be 
expressed  thus  :  These  ingenious  experiments  have  proved 
that  the  problem  of  fertilisation  must  not  be  studied,  as  has 
been  done  hitherto,  exclusively  by  morphological  methods, 
but  also  by  completely  new  methods  belonging  to  physico- 
chemical  science.  Only  in  this  way  shall  we  arrive  at  a  satis- 
factory insight  into  the  true  nature  of  the  fertilisation  and 
cleavage  of  the  ovum,  and  the  embryonic  development  that 
follows  these  processes.  For  the  present  we  have  no  certain 
information,  but  only  bold  speculations,  as  to  the  physico- 
chemical  factors  engaged  in  these  processes,  nor  do  we  know 
how  they  co-operate  mechanically  and  teleologically  to  accom- 

1  Das  Problem  der  Befruchtuny,  p.  47. 


plish  them.  The  naturalists  who  fancy  that  they  have  at 
last  succeeded  in  giving  a  purely  physico-chemical  explanation 
to  life  itself  are  doomed  to  disappointment. 


There  still  remains  one  class  of  phenomena  which  we  must 
consider  shortly,  because  it  throws  some  light  on  the  problem 
of  fertilisation,  namely,  artificial  fertilisation  of  non-nucleated 
fragments  of  ovum,  called  by  Y.  Delage  merogony.1  The 
first  experiments,  now  become  classical,  in  this  subject  were 
begun  in  1887  by  0.  and  K.  Hertwig,  and  continued  by  Boveri 
in  1889  and  1895.  They  resulted  in  the  surprising  discovery 
that  non-nucleated  fragments  of  sea-urchins'  ova  could, 
if  fertilised,  develop  to  the  larval  stage.  Others  have  subse- 
quently confirmed  this  discovery  by  means  of  experiments  on 
the  eggs  of  sea-urchins  and  other  animals  ;  we  may  mention 
particularly  Morgan  (1895),  Ziegler  (1896  and  1898),  and 
Delage  (1898,  1899  and  1901).  Similar  experiments  were 
made  by  Eawitz  in  1901  on  the  immature  eggs  of  holothurians, 
the  nucleus  of  which  is  unimportant  and  in  course  of  time 
disappears,  so  that  they  may  be  regarded  as  non-nucleate. 
In  1896-8  H.  E.  Ziegler  made  some  experiments  at  artificially 
constricting  sea-urchins'  eggs,  and  his  results  are  not  without 
bearing  on  the  question.2 

Experiments  in  merogony  have  been  made  with  plants 
also,  and  I  may  draw  attention  particularly  to  those  undertaken 
in  1901  by  Hans  Winkler  on  the  eggs  of  a  seaweed  (Cystosira).^ 
Let  us  now  examine  some  of  the  above-mentioned  experiments 
more  closely. 

Oskar  and  Eichard  Hertwig  succeeded  in  proving4  con- 
clusively that,  if  sea-urchins'  eggs  are  broken  by  shaking 
fragments  containing  no  nucleus  may  be  fertilised  by  the 

1  Of.  Korschelt  and  Heider,  Lehrbuch    der  vergl.  Entwicklungsgesch  ,  pp. 
149-151  and  625-626. 

2  A  full  list  of  the  works  to  which  I  have  referred  will  be  found  in  Korschelt 
and  Heider,  pp.  733-750. 

3  H.  Winkler,  '  tJber  Merogonie  und  Befruchtung  '  (Jahrbiicher  fur  wissen- 
schaftl.  Botanik,  XXXVI,  pp.  753-775). 

4  '  Uber  Befruchtungs-  und  Teilungsvorgange  des  tierischen  Eis '  (Jenaische 
Zeitschrift  fur  Naturwissenschaft,  XX,  1887). 


entrance  of  a  spermatozoon.  In  Boveri's  experiments,  such 
non-nucleated  fragments  of  the  ovum,  after  fertilisation  with 
one  spermatozoon  of  the  same  species,  developed  and  actually 
reached  the  stage  of  the  Pluteus  larva,  thus  showing  such  ova 
to  be  capable  of  normal  development.  In  this  way  Boveri 
obtained  dwarf  larvae,  larger  or  smaller  according  to  the  size 
of  the  fragment  of  ovum. 

The  experiments  made  by  Hertwig  and  Boveri  prove  that 
under  certain  conditions  the  sperm-nucleus  alone,  without 
the  egg-nucleus,  suffices  for  the  fertilisation  and  development 
of  the  animal  ovum,  in  exactly  the  same  way  as,  in  partheno- 
genesis, the  egg-nucleus  suffices  without  the  sperm-nucleus. 
Giard  called  this  phenomenon  simply  male  parthenogenesis,  as 
in  this  case  the  sperm-nucleus  receives  from  the  non-nucleate 
egg-cell  the  cytoplasm  necessary  for  its  development.  The 
same  idea  had  been  expressed  somewhat  differently  by  M. 
Verworn  in  1891,  and  in  1901  Kawitz  invented  the  name 
epliebogenesis  to  designate  the  process. 

The  embryos  of  the  non-nucleated  eggs  of  sea-urchins  only 
reached  the  stage  of  cleavage  into  sixteen  cells  in  Morgan's 
experiments,1  but  he  was  able  to  demonstrate  that  their  nuclei 
contained  only  half  the  normal  number  of  chromosomes 
(eleven  instead  of  twenty-two)  belonging  to  that  species.  It 
is  easy  to  see  why  this  is  so,  for  the  sperm-nuclei,  which  fer- 
tilised the  fragments  of  egg,  contained  the  reduced  number. 
This  fact  therefore  agrees  with  similar  phenomena  observed 
in  artificial  parthenogenesis  (see  p.  144),  and  shows  that  some- 
times half  the  normal  number  of  chromosomes  suffices  for  the 
embryonic  development  of  the  egg.  Whether  these  chromo- 
somes are  paternal  or  maternal  in  origin  is  immaterial  for  the 
purpose  of  embryonic  development,  but  not  for  that  of  heredity, 
as  Boveri's  next  experiments  show  with  a  degree  of  probability 
almost  amounting  to  certainty.3 

He  began  by  crossing  two  distinct  varieties  of  sea-urchin, 
and  fertilised  ova  of  Sphaerechinus  granularis  with  spermatozoa 
of  Echinus  microtuberculatus.  The  Pluteus  larvae  of  these 
two  species  can  easily  be  distinguished — those  of  Echinus  have 

1  '  The   fertilisation   of    non-nucleated   fragments   of    Echinoderm   eggs ' 
(Archiv  fur  Entwicklungsmechanik,  II,  1895). 

2  '  Ein  geschlechtlich  erzeugter  Organismus  ohne  miitterliche  Eigenschaften ' 
(Sitzungsberichte  der  Gesellschaft  fur  Morph.  und  Phys.,  Munich,  1889). 



a  much  more  slender  shape  and  a  different  formation  of  the 
calcareous  skeleton.  Boveri  succeeded  in  showing  that  the 
result  of  crossing  these  two  species  was  to  produce  hybrid 
larvae  (fig.  26)  occupying  a  position  midway  between  the  two 
larvae  of  pure  breed  (figs.  24  and  25)  and  displaying  a  mixture 
of  the  peculiarities  in  shape  of  both  parents. 

Boveri  next  proceeded  to  fertilise  ova  of  Sphaerechinus, 
partially  broken  by  shaking,  with  spermatozoa  of  Echinus. 
Of  the  larvae  produced  by  the  fragments,  some  showed  the 
hybrid  type,  and  Boveri  assumed  that  they  developed  either 
from  uninjured  ova,  or  from  fragments  containing  part  of  the 

FIG.  24. 

FIG.  25. 

FIG.  26. 

FIGS.  24-26.— Side  view  of  Pluteus  larvae:  FIG.  24  of  Echinus,  FIG.  25  of 
Sphaerechinus,  FIG.  26  of  the  hybrid  of  Sphaerechinus  <j?  and 
Echinus  3. 

From  Korschelt  and  Heider,  according  to  Boveri's  diagram. 

egg-nucleus,  into  which  a  spermatozoon  of  the  other  species 
had  found  its  way.  Other  larvae  were  particularly  small,  but 
displayed  the  pure  Echinus-type  (fig.  24).  Boveri  calls  these 
dwarf  Plutei,  and  believes  them  to  have  developed  from  non- 
nucleated  fragments  of  Sphaerechinus  ova,  and  therefore  to 
represent  altogether  the  paternal  Echinus-type,  because  the 
sperm-nucleus  fertilising  them  belonged  to  this  latter  species. 
According  to  Boveri's  view,  these  dwarf  Plutei  are  organisms 
without  any  maternal  characteristics,  and,  if  this  view  is  the 
true  one,  we  have  here  a  proof  that  the  cell-nucleus  does 
not  merely  determine  the  shape  of  the  embryo,  but  is  the  real 
bearer  of  heredity,  for  only  the  cell-nucleus  on  the  father's 
side,  and  not  the  egg-plasm  on  the  mother's  side,  stamped 


upon  the  embryo  its  specific  characteristics  as  a  pure  Echinus 

Boveri's  explanation  is  rendered  more  probable  by  the  fact 
that  the  dwarf  Plutei  of  the  Echinus  type  possessed  an  un- 
mistakably smaller  nucleus  than  larvae  of  the  same  size  of  the 
hybrid  type.  This  difference  in  the  size  of  the  nucleus  is  quite 
intelligible  if  we  may  assume  that  in  the  former  case  the  cell- 
nucleus  was  formed  from  only  one  pronucleus,  and  so  con- 
tained only  half  the  amount  of  chromatin,  whereas  in  the 
second  case  the  nucleus  was  produced  by  the  union  of  two 

Boveri  assumed,  therefore,  that  the  dwarf  larvae  of  pure 
Echinus-type,  produced  in  the  course  of  his  experiments  at 
cross-breeding,  really  developed  from  non-nucleated  fragments 
of  ovum,  and  consequently  were  organisms  devoid  of  any 
maternal  characteristics.  Morgan,  Seeliger,  Driesch  and  Delage 
have  brought  forward  a  number  of  objections  to  this  theory, 
but  Boveri  adheres  to  it  even  in  his  most  recent  works.  Yves 
Delage  himself  classes  Boveri's  experiments  among  what  he 
calls  experiences  decisives,  as  furnishing  evidence  of  great 
weight  in  the  solution  of  the  scientific  problem  under  dis- 
cussion. In  fact,  when  we  take  into  consideration,  firstly, 
that  non-nucleated  fragments  of  sea-urchins'  eggs  can  be 
fertilised,  and,  secondly,  that  Boveri  fertilised  them  with 
spermatozoa  of  another  species,  we  can  hardly  avoid  agreeing 
with  him  in  regarding  the  dwarf  larvae,  which  display  only 
paternal  characteristics',  as  the  products  of  non-nucleated  ova, 
deriving  from  the  father's  side  alone  their  nucleus,  and  con- 
sequently the  substance  which  bears  heredity. 

Quite  recently  E.  Godlewski  has  made  experiments l  at 
cross-breeding  between  sea-urchins  (Echinidae)  and  sea-lilies 
(Crinoidea),  by  fertilising  the  eggs  of  the  former  with  sper- 
matozoa of  the  latter,  and  the  results  which  he  obtained  are 
exactly  the  reverse  of  Boveri's.  All  the  hybrid  larvae  displayed 
purely  maternal,  and  no  paternal  characteristics,  even  in  cases 
where  a  non-nucleated  fragment  of  Echinus  ovum  was  fertilised 
with  an  Antedon  spermatozoon.  Godlewski  argues  from  this  that 
Boveri's  whole  morphological  theory  of  heredity  is  untenable, 

1  '  Untersuchungen  iiber  die  Bastardierung  der  Echiniden-  und  Crinoiden- 
familie  '  (Archiv  jiir  EntwicUungsmechanik,  XX,  1906,  pp.  579,  &c.). 


and  Verworn's  physiological  theory  must  be  substituted 
for  it ;  and  that  not  the  chromosomes,  but  the  egg-plasm, 
constitute  the  vehicle  of  transmission.  Such  far-reaching 
conclusions  need  confirmation  from  other  experiments  before 
they  can  be  accepted,  for  the  bulk  of  the  evidence  afforded 
by  biology  seems  to  show  decisively  that  the  chromosomes  of 
the  nucleus  are  the  material  bearers  of  heredity.  The  physio- 
logical fact  that  the  chromosomes  of  the  nucleus  and  the  proto- 
plasm of  the  egg  act  reciprocally  upon  one  another,  is  of  course 
included  as  a  fully  recognised  condition. 

The  successful  attempts  made  by  Boveri  and  others  to 
fertilise  non-nucleated  fragments  of  ova  show  that  under 
certain  circumstances  the  sperm-nucleus  alone  suffices  for  the 
development  of  the  egg.  But  this  statement  does  riot  imply 
that  it  is  the  sperm-nucleus  itself  which  gives  rise  to  the  process 
of  development :  it  may  be  the  sperm-centrosome  which  pene- 
trates into  the  egg  with  the  nucleus.  An  observation  made  by 
Boveri  in  1887  on  the  subject  of '  partial  fertilisation  '  suggests 
that  this  may  be  the  case.  He  saw  a  spermatozoon  enter  a 
sea-urchin's  egg.  Its  nucleus  remained  near  the  periphery 
of  the  egg,  whilst  the  centrosome  alone  with  its  amphiaster 
approached  the  egg-nucleus,  and  thereupon  the  first  cleavage- 
division  of  the  egg-nucleus  took  place.  The  sperm-nucleus 
united  with  one  of  the  daughter-nuclei  of  the  egg.  Wilson,  too, 
considers  that l  this  observation  affords  a  beautiful  illustration 
of  Boveri's  theory  that  it  is  the  centrosome  of  the  sperm-nucleus 
which  supplies  the  normal  stimulus  to  division  on  the  part  of 
the  ovum. 

Further  light  is  thrown  upon  this  interesting  question  by 
the  experiments  made  by  H.  E.  Ziegler  in  1896  and  1898  on 
sea-urchins'  eggs,  which  he  fertilised  artificially  and  then 
divided  by  constricting  them  with  fine  threads.3 

In  every  case  in  which  the  egg  was  so  divided  that  the 
sperm-nucleus,  with  its  centrosome  and  centrosphere,  was 
contained  in  one-half  of  the  egg,  and  the  egg-nucleus  in  the 
other  half,  the  former  half  divided  in  the  ordinary  manner, 
whereas  an  aster  was  formed  near  the  egg-nucleus,  and  all 

1  The  Cell,  p.  190. 

2  Cf.  H.  E.  Ziegler,  *  Experimented  Studien  iiber  die  Zellteilung  :   I.  Die 
Zerschniirung  der  Seeigeleier ;    II.    Furchung  ohne   Chromosomen '   (Archiv 
fur  Entwicklungsmechanik,  VI,  1898,  Part  2,  pp.  249-293). 


preparations  were  made  for  cell-division,  which,  however,  never 
actually  took  place.  These  experiments  seem  to  show  again 
that,  in  normal  fertilisation,  it  is  the  sperm-centrosome  that 
renders  the  egg-nucleus  capable  of  active  division.  In  some 
experiments  made  in  1897  and  1901,  Boveri  broke  up  some 
sea-urchins'  eggs  after  fertilisation,  and  found  asters,  leading 
in  some  cases  to  cell- divisions,  also  in  fragments  containing 
only  egg-nucleus,  and  no  particle  of  the  sperm-nucleus  or  its 
centrosome.  Wilson,  Winkler,  and  others  are  inclined  to 
explain  this  last  phenomenon  by  assuming  that,  as  soon  as 
the  spermatozoon  enters  the  egg,  its  centrosome  sets  up  a 
kind  of  fermentation  l  in  the  whole  egg-plasm,  so  that  even  the 
parts  remote  from  the  centrosome  are  stimulated  to  division. 
This  explanation  would  bring  us  back  to  the  chemical  side  of 
the  problem  of  fertilisation,  and,  as  was  said  on  p.  148,  we 
cannot  do  more  at  present  than  advance  some  vague  specula- 
tions on  the  subject. 

The  experiments  in  merogony  suggest  this  question :  Is 
it  possible  that  the  sperm-centrosome  alone,  without  the 
sperm -nucleus  and  without  the  egg-nucleus,  has  the  power 
of  setting  up  a  regular  process  of  division  and  so  of  beginning 
embryonic  development  in  the  fragments  of  ovum  ? 

In  1897  Boveri  made  an  experiment  3  and  fertilised  some 
non-nucleated  fragments  of  Echinus  eggs  with  spermatozoa 
of  another  species  (Strongylocentrotus).  It  happened  that  the 
whole  nuclear  substance  of  both  nuclei  passed  into  one  half 
of  the  egg,  and  the  centrosome  alone  into  the  other.  The 
former  half  divided  in  the  regular  way,  but  in  the  other  a 
series  of  divisions  took  place  in  the  centrosomes  and  attraction 
spheres,  but  no  cell-division  "occurred.  This  observation  led 
Boveri  to  conclude  that,  at  any  rate  for  sea-urchins,  at  least 
one  nucleus  is  indispensable  for  cell- division.  H.  E.  Ziegler, 
however,  believes  that  he  succeeded  in  1898  in  effecting  a 
'  cleavage  without  chromosomes.'  In  an  egg  of  Echinus 
microtuberculatus,  fertilised  with  spermatozoa  of  the  same 
species,  at  the  first  division  the  entire  nuclear  substance  of  both 
the  sexual  nuclei  passed  into  one  of  the  cells  formed  by  division, 

1  Cf.  Korschelt  and  Heider,  Lehrbuch  der  vergl.  Entwicklungsgesch.,  pp. 

2  *  Zur  Physiologie  der  Kern-  und  Zellteilung  '  (Sitzungsberichte  d.  physilc.- 
mediz.  Gesellsch.,  WErzburg). 


whilst  a  centrosome  with  its  centrosphere  was  left  in  the  other. 
The  cell  containing  the  nuclei  divided  with  perfect  regularity, 
but  also  in  the  non-nucleated  cell  a  series  of  cleavages  took 
place  in  the  cell-body  ;  they  were,  however,  incomplete  and 
irregular.  It  is  unfortunate  that  in  this  interesting  experi- 
ment Ziegler  did  not  use  nuclear  stains,  but  only  treatment 
with  acetates,  to  prove  that  there  was  really  no  chromatin 
present  in  the  division-cell  that  apparently  contained  no 
chromosomes.  This  flaw  has  left  the  matter  still  doubtful. 

My  own  opinion  is  that,  in  these  instances  of  merogony  also, 
the  centrosome  is  a  biomechanical  instrument  for  assisting 
nuclear  division,  but  is  not  an  independent  division-organ  of 
the  cell.  It  is  true  that  the  experiments  described  above 
confirm  Boveri's  opinion  (cf.  p.  126),  that  in  the  case  of  most 
animal  ova  the  centrosome  of  the  spermatozoon  gives  the 
immediate  impulse  to  cell-division  in  the  normal  course  of 
fertilisation,  but  it  is  not  absolutely  indispensable  to  the 
beginning  of  the  process  of  embryonic  development.  This  is 
proved  by  the  phenomena  of  natural  and  artificial  partheno- 
genesis (see  pp.  135  and  139),  where  no  male  centrosome  can 
possibly  be  present.  Moreover,  many  circumstances  to  which 
I  have  referred  (see  p.  143)  suggest  the  idea  that  centrosomes 
are  not  permanent  organs  in  the  cell,  but  are  formed  afresh 
in  the  egg-plasm  as  occasion  requires. 


(See  Plate  II) 

We  have  now  completed  our  examination  of  the  relations 
in  which  cell-division  stands  to  the  problems  of  fertilisation 
and  heredity.  The  facts  to  be  taken  into  account  are  so 
numerous  and  of  so  many  kinds,  and  the  interpretations  put 
upon  them  are  so  varied,  that  it  is  naturally  no  easy  task  to 
draw  from  them  any  clear  and  definite  conclusions.  We 
might  almost  say  that  we  cannot  see  the  wood  because  of  the 
trees  in  it !  And  yet  the  wood  is  one  whole,  composed  of  the 
trees  which  various  naturalists  have  laboriously  planted  and 
cultivated.  And  there  are  some  paths  through  it,  though 


they  are  still  footways  and  not  carriage  drives,  for  the  wood  is 
still  wild,  and  not  a  park. 

Let  us  try  now  to  follow  these  paths  by  surveying  the  facts 
once  more  and  seeing  in  what  respects  they  conform  to  general 
laws.  We  must  be  on  our  guard  against  adopting  the  methods 
of  those  theorists  who  simply  cast  aside  and  reject  all  that 
does  not  coincide  with  their  subjective  ideas. 

Both  the  male  and  the  female  germ-cells  prepare  for  their 
union  in  the  process  of  fertilisation  by  a  double  maturation- 
division.  These  preparatory  divisions  cause  a  reduction  in  the 
number  of  chromosomes  (if  it  has  not  taken  place  before), 
so  that  the  cells  contain  only  half  the  normal  number  contained 
in  the  somatic  cells  of  the  same  species.  The  act  of  fertilisa- 
tion restores  the  number  to  the  normal,  as  the  chromosomes 
of  the  male  and  female  pronuclei  meet  in  the  cleavage-spindle 
of  the  ovum,  and  by  splitting  lengthwise  furnish  an  equal 
number  of  paternal  and  maternal  chromosomes  for  the  daughter- 
nuclei  of  the  ovum  in  process  of  cleavage. 

Normal  fertilisation  has  as  its  essential  feature  the  union 
of  two  germ-cells,  one  being  male  and  the  other  female,  and 
the  union  is  more  especially  a  union  of  their  nuclei.  E.  B. 
Wilson  sums  up  this  result  on  p.  230  of  his  excellent  work 
'  The  Cell '  (1902)  in  the  following  words  :  '  We  thus  find  the 
essential  fact  of  fertilisation  and  sexual  reproduction  to  be  a 
union  of  equivalent  nuclei ;  and  to  this  all  other  processes  are 
tributary.'  This  is  true  both  of  the  animal  and  of  the  vegetable 
kingdom.  With  reference  to  the  latter  Wilson  says  (p.  216)  : 
'  The  essential  fact  is  everywhere  a  union  of  two  germ-nuclei — 
a  process  agreeing  fundamentally  with  that  observed  in  animals.' 
Eichard  Hertwig  uses  similar  language  in  the  seventh  edition 
of  his  '  Lehrbuch  der  Zoologie,'  1905,  p.  124  (Eng.  trans,  p.  149) : 
'  Since  not  until  this  point  (i.e.  the  union  of  the  sexual  nuclei) 
is  fertilisation  complete,  we  arrive  at  the  fundamentally 
important  proposition  that  the  essential  feature  of  fertilisation 
consists  in  the  union  of  egg-  and  sperm-  nuclei.' 

Nuclear  union  can,  however,  assume  various  forms.  It 
may — as  in  the  Echinus-type — lead  to  the  formation  of  a 
resting  cleavage-nucleus,  in  which  the  chromosomes  of  the 
two  pronuclei  are  already  brought  into  contact,  or — as  in  the 
^4scans-type — the  two  pronuclei  may  remain  apart,  so  that 


their  chromosomes  are  not  grouped  in  a  common  division- 
figure  until  the  cleavage-spindle  is  formed.  Moreover,  the 
part  played  by  the  centrosomes  in  the  processes  of  fertilisation 
varies.  In  normal  fertilisation  of  the  animal  ovum,  the  male 
centrosome  acts  as  an  organ  of  division,  inducing  the  formation 
of  the  cleavage-spindle,  but  no  centrosomes  have  been  observed 
in  the  fertilisation  of  the  higher  orders  of  plants.  In  many 
animal  ova  (e.g.  Myzostoma,  according  to  Wheeler)  the  place 
of  the  sperm-centrosome  as  an  organ  of  division  seems  to  be 
taken  by  the  oocentrosome.  Finally,  in  physiological  super- 
fecundation  among  animals  and  in  double-fertilisation  among 
angiosperms  in  the  vegetable  kingdom,  not  only  one  sperm- 
nucleus,  but  two  or  more,  are  concerned  in  the  process  of 
fertilisation,  although  only  one,  which  unites  with  the  egg- 
nucleus,  has  a  distinctly  generative  function,  the  duty  assigned 
to  the  others  being  rather  of  a  vegetative  character,  and  con- 
sisting of  the  formation  of  nourishment  for  the  embryo. 

So  far  we  have  spoken  only  of  the  usual  case  in  which  two 
nuclei,  the  male  and  female  pronuclei,  carry  on  the  fertilising 
process  in  the  ovum.  Analogous  to  this  are  the  phenomena 
of  conjugation  which  occur  in  unicellular  organisms.  But  in 
artificial  fertilisation  of  non-nucleated  fragments  of  ovum, 
only  the  sperm-nucleus  is  concerned,  and  in  animal  eggs  this 
is  generally  accompanied  by  a  sperm-centrosome. 

In  parthenogenetic  development  of  the  ovum  there  is  no 
fertilisation  by  a  spermatozoon,  but  the  process  is  carried  on 
by  the  egg-nucleus  alone  ;  in  natural  parthenogenesis  it  is 
assisted  by  the  oocentrosome,  and  in  artificial  parthenogenesis 
by  centrosomes  newly  formed  in  the  egg-plasm  by  means  of 
exterior  agents.  WasiliefT  considers  that  even  these  centro- 
somes may  be  absent.  The  centrosome  alone,  without  either 
egg-  or  sperm-nucleus,  seems  to  be  able  to  begin  the  process  of 
cell-division,  but  not  to  succeed  in  carrying  it  through. 

Let  us  now  sum  up  the  results  of  these  observations  and 
experiments.1  It  seems  safe  to  infer  from  them  that  the  nucleus 
of  the  germ-cell  is  of  primary  importance  in  normal  fertilisation, 
as  well  as  in  artificial  fertilisation  of  non-nucleated  fragments 
of  ova,  and  in  parthenogenesis.  Opinions  are  still  divided 

1  Cf.  on  this  subject  Korschelt  and  Heider,  Lehrbuch  der  vergl.  Entwick- 
lungsgesch,,  pp.  697-706  ('  Wesen  und  Bedeutung  der  Befruchtung  '). 


as  to  the  centrosomes,  whether  they  originate  in  the  achro- 
matic nuclear  substance  or  in  the  egg-plasm ;  they  seem 
to  me  to  be  of  secondary  importance  as  merely  assisting  the 
division  of  nucleus  and  cell.  That  the  egg-plasm  is  an  essential 
factor  in  the  processes  of  fertilisation  and  development  is 
proved  beyond  question,  especially  by  the  phenomena  of 
artificial  parthenogenesis,  which  gave  rise  to  the  modern 
chemico-physical  theories  regarding  fertilisation. 

What,  then,  is  the  answer  to  the  question  raised  by  Aristotle, 
and  repeated  from  age  to  age  in  the  course  of  the  dispute 
between  ovulists  and  animalculists  :  *  Is  the  essence  of  the 
animal  and  vegetable  species  contained  in  the  egg-cell  or 
in  the  sperm-cell  ?  ' l 

Many  facts,  and  especially  the  phenomena  of  natural  and 
artificial  parthenogenesis  (see  pp.  135, 139,  &c.)  seem  to  support 
Aristotle's  opinion  that  the  material  required  to  form  the  new 
individual  is  all  contained  in  the  egg-cell,  and  that  the  sperm- 
cell  only  supplies  the  stimulus  causing  this  material  to  develop.3 

In  a  modernised  form  this  opinion  is  revived  in  Boveri's 
theory  of  fertilisation,  which  regards  the  ovum  as  a  complete 
piece  of  clockwork,  lacking  only  the  mainspring,  or  rather, 
lacking  only  the  key  to  wind  up  the  mainspring.  This  key 
is  the  sperm-centrosome,  that  sets  in  action  the  dividing 
process  of  the  ovum.  The  same  fundamental  idea  is  present  in 
Delage's  chemico-physical  theory  of  fertilisation,  according 
to  which  the  mature  but  unfertilised  egg-cell  is  in  a  state  of 
unstable  equilibrium  ;  this  equilibrium  is  disturbed  by  a 
reduction  in  the  water  of  the  egg-plasm,  caused  by  the  entrance 
of  the  spermatozoon,  and  the  ovum  is  thus  stimulated  to 
independent  development. 

Other  considerations  of  no  less  weight  are  directly  opposed 
to  the  theory  that  the  egg-cell  alone  contains  the  essence  of  the 
new  individual.  The  experiments  in  artificial  impregnation  of 
non-nucleated  fragments  of  ova,  and  especially  the  results 
obtained  by  Boveri  (see  p.  150),  show  that  the  sperm-nucleus 
alone — just  as  in  parthenogenesis,  the  egg-nucleus  alone — in 
conjunction  with  the  egg-plasm,  is  able  to  cause  the  egg  to 

1  Cf.  p.  104.     See  also  0.  Hertwig,  Allgemeine  Biologie,  p.  352. 

2  Cf.  Aristotle,  *  De  animalium  generatione,'  cap.  2  (Aristotdis  opera  omnia, 
ed.  Didot,  III,  320).     Aristotle  docs  not  of  course  speak  of  the  elements  of 
reproduction  as  cellular,  for  he  had  no  knowledge  of  cells  at  all. 


produce  a  new  individual  of  the  species  concerned.  The 
embryonic  material  for  the  formation  of  the  new  individual 
must  therefore  be  contained  as  completely  in  the  nuclear 
substance  of  the  spermatozoon  as  in  that  of  the  ovum.  The 
nuclei  of  both  germ-cells  have  then  with  regard  to  the  develop- 
ment of  the  embryo  the  same  prospective  potency,  as  Driesch 
calls  it. 

Let  us  now  turn  to  a  third  series  of  phenomena, 
viz.  to  the  facts  of  normal  fertilisation,  which  are  of  great 
importance  for  our  purpose  (cf.  pp.  119,  &c.).  We  have  already 
seen  that  the  process  of  fertilisation  culminates  in  the  union  of 
the  nuclei  of  the  two  germ-cells,  and  that  the  originally  insigni- 
ficant sperm-nucleus  finally  becomes  exactly  equivalent  to 
the  egg-nucleus  in  size  and  shape  and  in  number  of  chromo- 
somes. The  sperm-nucleus  supplies  for  the  development 
of  the  new  individual  exactly  the  same  amount  of  chromatin 
nuclear  substance  as  the  egg-nucleus  ;  the  nuclear  substance  of 
the  cleavage-spindle  of  the  embryo  represents  the  sum  of  that 
contained  in  the  nuclei  of  the  ovum  and  spermatozoon ;  the 
essence  of  the  animal  or  vegetable  species,  as  propagated  by 
normal  fertilisation,  is  therefore  first  contained  in  the  sum  of  the 
chromatin  nuclear  substance  of  the  male  and  female  pronuclei, 
and  the  essence  of  normal  fertilisation  culminates  therefore  in  the 
union  of  the  chromosomes  of  both  to  form  one  new  cell-nucleus.1 
In  his  'Allgemeine  Biologie'  (1906),  p.  301,  0.  Hertwig  states 
his  conclusions  in  the  following  words  :  '  The  nuclear  sub- 
stances supplied  in  exactly  equal  quantities  by  two  distinct 
individuals  are  the  especially  active  materials,  the  union  of 
which  is  the  chief  object  of  the  act  of  fertilisation ;  they  are 
the  real  materials  of  fertilisation.'  3 

We  cannot  avoid  asking  further  questions  :  What  is  the 
object  of  this  union  of  paternal  and  maternal  nuclear  elements 
in  the  normal  course  of  fertilisation  ?  Is  it  not  altogether 
superfluous,  if  what  is  essential  to  the  species  is  contained 

1  This  explains  why  the  number  of  chromosomes  in  the  somatic  cells  of 
animals  and  plants  that  are  propagated  by  sexual  reproduction  is  always  even. 
Cf.  Chapter  V,  p.  92. 

2  A  detailed  proof  that  the  nucleus  is  the  physical  basis  of  inheritance 
is  given  by  Hertwig  in  the  thirteenth  chapter  of  the  same  work,  pp.  354-363. 
His  proof  depends  upon  four  kinds  of  evidence,  which  agree  on  the  whole  with 
those  that  I  have  adduced. 


completely  either  in  the  egg-cell  alone,  or  in  the  sperm-cell  alone  ? 
What  is  the  use  of  the  vast  difference  between  the  ovum  and 
the  spermatozoon  in  the  higher  organisms,  where  the  former 
is  very  large  and  richly  provided  with  nutritive  plasm,  and 
the  latter  is  diminutive  and  consists  of  a  thread  of  cytoplasm 
by  way  of  tail,  a  head  containing  a  nucleus,  and  a  middle-piece  ? 
What  is  the  use  of  the  complicated  maturation- divisions,  by 
which  the  egg-cell  and  the  sperm-cell  prepare  for  their  future 
union  in  the  process  of  fertilisation  ?  What  is  the  good  of 
all  these  complicated  arrangements  ?  Are  they  not  perfectly 
aimless  ? 

It  is  true  that  the  two  kinds  of  germ-cells  are  in  their  origin 
essentially  alike.  This  is  proved,  on  the  one  hand,  by  the 
embryonic  development  of  the  individual,  in  which  the  egg- 
and  sperm-cells  proceed  from  similar  germinal  Anlagen  and  are 
differentiated  only  at  subsequent  stages  of  development.  It 
is  proved,  on  the  other  hand,  by  the  phenomena  of  conjugation 
in  unicellular  organisms,  in  which  isogamy,  i.e.  the  union  of 
two  similar  germ-cells,  represents  theoretically  and  practically 
the  first  condition  of  propagation  by  germ-cells  (see  p.  132  on 
Pandorina  monim).  Nevertheless,  the  differentiation  of  the 
male  and  female  germ-cells  in  the  organic  kingdom,  and  their 
union  in  the  normal  course  of  fertilisation,  are  processes  of  the 
highest  teleological  significance. 

In  order  to  see  this  more  clearly,  we  must  follow  Boveri, 
Weismann,  E.  Hertwig,  Y.  Delage,  &c.,  in  recognising  a  two- 
fold object  in  fertilisation.  (1)  It  aims  at  inciting  to  develop 
a  new  individual,  and  (2)  it  aims  at  transmitting  the  combined 
properties  of  both  parents  to  this  individual. 

1.  The  first  of  these  two  aims  can  be  realised  both  among 
animals  and  plants  by  other  means  besides  fertilisation.  We 
have  seen  this  in  the  case  of  Infusorians  and  other  unicellular 
organisms,  which  increase  either  by  simply  splitting  in  two,  or 
by  breaking  up  a  colony  of  cells  into  single  cells.  Although 
with  them  from  time  to  time  periods  of  conjugation  have  to  inter- 
vene between  the  periods  of  non-sexual  or  agamous  multiplica- 
tion, E.  Hertwig's  recent  observations  seem  to  show  that  there 
is  no  direct  connexion  between  conjugation  and  the  multiplica- 
tion of  individuals  by  division.  In  multicellular  animals  and 
plants,  which  are  propagated  by  gemmation,  we  noticed  that 


the  new  individuals  come  into  existence  independently  of  any 
process  of  fertilisation.  This  is  seen  still  more  plainly  in  the 
case  of  plants  that  can  be  propagated  indefinitely  by  means  of 
cuttings  and  tubers,  without  weakening  their  growth,  such  as 
the  grape-vine  and  the  potato.  The  absolutely  sexless  pro- 
pagation of  Laminaria  and  other  plants  bears  witness  to  the 
same  fact,  and  natural  parthenogenesis  in  animals  and  plants 
shows  that  the  development  of  a  new  individual  from  an  egg 
is  not  necessarily  connected  with  its  fertilisation. 

In  spite  of  all  this,  however,  it  cannot  be  denied  that 
where  the  normal  process  of  fertilisation  is  the  rule,  it  is  of 
great,  even  of  essential,  importance  in  realising  the  first  of  the 
two  aims  of  fertilisation,  viz.  in  stimulating  the  formation  of 
a  new  individual. 

According  to  Biitschli,  the  organic  substance  requires  a 
periodical  rejuvenescence  of  its  vital  powers.  The  capacity 
for  growth  and  multiplication  of  cells  is  gradually  weakened 
and  exhausted  as  life  goes  on,  and  eventually  death  from 
senile  decay  must  follow.  But  in  order  that  the  species  may 
not  perish  with  the  individual,  it  is  necessary  that  certain  cells, 
viz.  the  germ-cells,  of  one  individual  should  unite  with  those 
of  another  in  the  process  of  fertilisation,  that  thereby  their 
vital  force  may  be  regenerated  and  renewed.  There  is  certainly 
much  truth  in  this  theory,  although  it  has  been  vigorously 
contested  by  Weismann  in  his  '  Lectures  on  the  Evolution 
Theory  '  (vol.  i.  pp.  325-328,  English  translation).  His  germ- 
plasm  theory  leads  Weismann  to  regard  the  germ-cells  as 
'  potentially  immortal,'  and  so  he  thinks  there  can  be,  in 
connexion  with  them,  no  suggestion  of  senile  decay  calling  for 
rejuvenescence.  But  even  Weismann  does  not  venture  to 
deny  that  a  strengthening  of  the  metabolism  or  constitution  of 
the  germ-cells  is  connected  with  fertilisation,  and  this  differs 
very  little  from  an  actual  rejuvenescence  of  their  vital  force. 
This  is  the  reason  why  E.  Hertwig1  has  recently  adopted 

1  '  Uber  Wesen  und  Bedeutung  der  Befruchtung  '  (Sitzungsberichte  der 
Akad.  der  Wissenschaften,  Munich,  XXXII,  1902,  pp.  57-73) ;  *  Uber  Korrela- 
tion  von  Zell-  und  Kerngrosse  und  ihre  Bedeutung  fur  die  geschlechtliche 
Differenzierung  und  die  Teilung  der  Zelle '  (Biolog.  Zentralblatt,  1903,  Nos. 
2  and  3) ;  '  tJber  das  Wechselverhaltnis  von  Kern  und  Protoplasma,'  Munich, 
1903  (reprinted  from  the  Munchener  Medizin,  Wochenschri/t,  I) ;  '  Uber  das 
Problem  der  sexuellen  Differenzierung '  ( Verhandl.  der  Deutschen  Zoolog. 
Gesellsch.,  1905,  pp.  186-214). 


Biitschli's  theory  under  a  somewhat  modified  form,  and  with 
fresh  evidence  in  support  of  it.  Hertwig  sees  in  the  conjugation 
processes  of  unicellular  organisms,  and  in  the  phenomena 
of  fertilisation  in  multicellular,  an  important  reorganisation 
of  their  organic  substance,  and  he  lays  particular  stress  upon 
the  restoration,  by  these  means,  of  that  relation  between 
nucleus  and  cytoplasm  in  the  cell  which  is  best  adapted  for 
carrying  on  the  vital  functions. 

Fr.  Schaudinn's  opinion  approximates  closely  to  Hertwig's.1 
He  thinks  that  the  object  of  fertilisation  is  to  restore  the 
proper  equilibrium  between  the  vegetative  and  animal  proper- 
ties of  the  organism  ;  he  regards  the  egg-cell  as  the  principal 
bearer  of  the  vegetative,  and  the  sperm-cell  as  that  of  the 
animal  properties,  because  in  the  former  the  cytoplasm,  and  in 
the  latter  the  nucleus,  predominates. 

Whilst  B.  Hertwig  and  Fr.  Schaudinn  in  their  theories 
emphasise  particularly  the  physiological  interaction  of  the 
various  constituents  of.  the  cell,  A.  Buhler2has  based  a  rejuj- 
venescence  theory  of  his  own  upon  the  chemical  nature  of  the 
metabolism  in  the  living  cells.  He  sums  it  up  in  the  following 
words  : '  I  have  therefore  arrived  at  the  conclusion  that,  through 
the  act  of  fertilisation,  something  is  again  imparted  to  the  new 
organism  which  the  old  organism  gradually  lost  in  life  and 
through  the  processes  of  life,  until  its  eventual  death  ;  this 
something  being  a  molecular  constitution  of  its  parts,  rendering 
them  capable  of  metabolism,  and  so  fit  to  underlie  all  the  vital 

From  what  has  been  already  said  on  the  subject  it  is 
quite  clear  that  there  are  very  various  views  regarding  the 
rejuvenescence  of  the  capacity  of  the  germ-cells  to  develop, 
especially  in  normal  fertilisation.  Let  us  therefore  return  to 
the  consideration  of  some  of  these  theories. 

According  to  Boveri  and  Strasburger,  the  centrosome  of  the 
spermatozoon  supplies  the  egg-cell  with  fresh  kinoplasm, 
whilst  the  trophoplasm  of  the  egg-cell  assists  the  sperm- 
nucleus  to  develop.  According  to  Y.  Delage,  the  sperm- 

1  '  Neue  Forschungen  iiber  die  Befruchtung  bei  Protozoen '  ( Verhandl.  der 
Deutschfin  Zoolog.  Gesellsch.,  1905,  pp.  16-35.  and  especially  p.  33). 

2  '  Alter  und  Tod  ;   eine  Theorie  der  Befruchtung  '  (Biolocj.  Zentralblatt, 
XXIV,  1904,  Nos.  2,  3  and  4). 


nucleus  renews  the  developing  capacity  of  the  egg-cell,  by 
taking  away  water  from  the  egg-plasm,  whilst  the  sperm- 
nucleus  grows  into  the  male  pronucleus  precisely  by  absorbing 
this  water. 

We  must  not  overlook  the  fact  that  a  rejuvenescence  of 
the  developing  capacity  is  probably  connected  with  the  matura- 
tion-divisions of  the  germ-cells,  but  it  presupposes  the  reunion 
of  the  reduced  and  consequently  rejuvenated  nuclear  substance 
of  both  cells  in  the  process  of  fertilisation,  for  the  formation  of  a 
new  and  particularly  vigorous  nucleus.  If  this  union  is  not 
effected,  both  cells  generally  perish,  and  no  further  develop- 
ment results. 

This  seems  to  point  to  the  fact  that  the  nuclear  union  of  the 
two  germ-cells  in  fertilisation  must  have  some  other,  higher 
purpose  than  the  mere  renewal  of  vital  capacity  in  the  single 
germ-cells,  for  the  differentiation  of  the  germ-cells  into  egg- 
and  sperm-cells,  and  the  physiological  division  of  labour 
connected  with  this  differentiation,  and  the  maturation- 
divisions  of  both  germ-cells  all  result  in  this — the  egg-cell 
alone  and  the  sperm-cell  alone  are  made  incapable  of  further 
independent  development ;  the  new  life  of  the  embryo  has  to 
proceed  from  their  union.  The  object  of  this  union  is  the 
second  of  the  objects  of  fertilisation  that  we  have  already 
mentioned,  viz.  the  transmission  to  the  offspring  of  the  com- 
bined properties  of  both  parents.1 

2.  This  aim  can  in  fact  be  attained  only  by  fertilisation, 
in  the  case  of  the  higher  organisms,  or  by  the  corresponding 
processes  of  conjugation,  in  that  of  the  lower  organisms.  In 
agamous  propagation  the  properties  of  one  individual  only 
can  be  transmitted  to  its  offspring,  and  the  same  is  true  of 
unisexual  propagation.  The  egg-cell  that  develops  partheno- 
genetically  can  transmit  only  maternal  qualities  to  the  new 
creature,  and  in  the  same  way,  if  we  accept  Boveri's  observa- 
tions on  this  subject  as  evidence,  when  non-nucleated  egg- 
fragments  are  fertilised,  only  the  paternal  sperm-nucleus  is 
the  bearer  of  heredity.  But  in  normal  fertilisation,  on  the 
contrary,  both  parents'  properties,  united  or  blended, 

1  I  need  hardly  point  out  that  heredity  is  not  in  itself  part  of  fertilisation, 
for  this  is  plain  from  the  cases  of  non-sexual  or  unisexual  propagation.  Cf. 
Reinke,  Einleitung  in  die  theoretische  Biologie,  pp.  413,  414. 

M  2 


are  transmitted  to  their  offspring.  This  end  is  served  by 
all  the  morphological  and  physiological  processes  in  the 
germ-cells  that  prepare  them  for  fertilisation,  or  take  place 
during  it. 

All  modern  cytologists  are  agreed  in  regarding  the  reduction 
in  the  number  of  chromosomes  in  the  mature  germ-cells,  the 
restoration  of  the  original  number  by  the  union  of  the  chromo- 
somes in  the  male  and  female  pronuclei,  and  the  even  distribu- 
tion of  the  paternal  and  maternal  chromosomes  at  the  cleavage 
of  the  fertilised  ovum,  as  constituting  a  process  of  great  regula- 
tive importance,  to  which  we  must  ascribe  an  eminently 
final  teleological  significance,  as  do  E.  B.  Wilson1  and  J. 
Beinke.3  Let  us  begin  by  forming  a  clear  idea  of  the  process 
of  reduction  by  which  the  number  of  chromosomes  in  the 
germ-cells  is  reduced  to  half.3 

The  reason  for  this  is  given  by  Weismann4  and  by  Oskar 
Hertwig5  and  others  ;  it  is  a  process  to  prevent  a  summation 
of  the  hereditary  substances.  Let  n  represent  the  number 
of  chromosomes  constantly  present  in  the  somatic  cells  of 
any  definite  species  of  animal  or  plant ;  if  no  reduction  took 
place  before  fertilisation,  the  fertilised  ovum  and  the  somatic 
cells  developed  from  it  in  the  next  generation  would  each 
contain  2n  chromosomes,  and  the  number  would  go  on  increas- 
ing for  ever  in  geometrical  progression.  As  the  chromosomes 
of  each  species  have  a  definite  maximum  size,  fluctuating  it  19 
true  within  certain  limits,  it  follows  that  in  course  of  time 
either  all  the  somatic  cells  would  consist  exclusively  of  chromo- 
somes, or  the  size  of  the  cells,  and  consequently  of  the  body 
of  the  individual,  would  attain  such  huge  dimensions,  that 
there  would  be  no  room  for  them  in  the  world.  Both  con- 
clusions are  obviously  absurd  and  quite  chimerical.  In  the 
first  case  we  should  have  creatures  more  preposterously  con- 
structed than  the  fabulous  Hydra,  which  consisted  entirely  of 
|  heads.  In  the  second  case  we  should  have  giants  whose  heads 
I  would  touch  the  moon.  Therefore,  some  kind  of  regular 

1  The  Cell,  1902,  chapter  v,  pp.  233-288. 

2  Einleitung  in  die  theoretische  Biologic,  p.  442. 

3  See  pp.  110,  &c. ;  cf.  also  Korschelt  and  Heider,  Lehrbuch  der  vergl.  Ent- 
wicklungsgesch.,  pp.    606-713     ('Wesen    und    Bedeutung     der    Chromatin- 
reduktion  '). 

4  Lectures  on  the  Evolution  Theory,  I,  pp.  303,  &c.,  Eng.  trans. 

6  Die  Zelle  und  die  Qewebe,  I,  Jena,  1892  ;  II,  Jena,  1898,  chapter  9. 


reduction  in  the  number  of  chromosomes  in  the  germ-cells 
may  be  described  as  absolutely  necessary. 

But  it  would  be  quite  possible  for  the  numerical  reduction, 
accompanied  by  a  corresponding  quantitative  diminution  in 
the  amount  of  chromatin,  to  be  effected  in  some  other  way 
than  that  in  which  it  actually  occurs  in  the  reduction  processes 
preparatory  to  fertilisation.  It  might  take  place  after  fertilisa- 
tion by  means  of  some  regulative  process,  causing  some  of 
the  chromosomes  to  dissolve  and  be  incorporated  with  the 
protoplasm  of  the  cell.  This  consideration  has  led  Weismann, 
0.  Hertwig  and  others  to  conjecture  that  the  processes  of 
reduction  aim  at  the  elimination  of  important  factors  in 
organisation ;  Weismann  goes  so  far  as  to  think  that  by 
the  reduction  of  chromosomes  definite  '  ancestral  plasms ' 
are  eliminated  from  the  parental  germ- cells.  In  other  words, 
according  to  these  authors,  whose  views  are  now  almost 
universally  accepted,  numerical  and  quantitative  reduction 
of  the  chromatin  is  connected  with  a  qualitative  reduction.1 
There  is,  however,  great  diversity  of  opinion  as  to  the  way 
in  which  this  is  effected  and  its  real  significance.  There  are 
even  a  few  naturalists  who,  like  Yves  Delage,2  absolutely 
question  the  expediency  and  the  necessity  of  any  such 
qualitative  reduction  of  the  chromatin. 

In  spite  of  all  these  and  many  other  difficulties  and  objec- 
tions, we  cannot  avoid  regarding  as  of  great  teleological 
importance  the  fact  that,  before  normal  fertilisation,  the 
number  of  chromosomes  in  the  germ-cells  is  regularly  reduced 
to  half,  and  then  is  brought  up  again  to  the  normal  by  means 
of  fertilisation.  The  maturation  processes  in  the  germ-cells 
take  place  unmistakably  in  view  of  subsequent  fertilisation. 
Independently  of  it,  they  would  be  perfectly  aimless,  if  not 
actually  harmful,  because  they  render  the  egg-cell  incapable 
of  further  division,  and  so  condemn  it  to  death,  if  no  fertilisa- 
tion follows.  This  is  still  more  true  of  the  spermatozoon, 
which  in  its  whole  structure  is  simply  designed  to  be  able 
to  fertilise  an  egg-cell.  There  must  be  some  deeply  signi- 
ficant purpose  hidden  under  these  phenomena,  and  it  is  this  : 
The  union  of  the  nuclear  substances  of  the  egg-  and  sperm-cell 

1  Cf.  Korschelt  and  Heider,  pp.  149,  712,  &c. 

2  Lea  theories  de  la  fecondation,  p.  131. 


renders  possible  the  transmission  to  the  offspring  of  the 
properties  of  both  parents.  The  transmission  of  the  combined 
properties  is  effected  in  a  very  sure  and  simple  way  by  the 
reduction  in  the  number  of  chromosomes  in  the  two  pronuclei, 
by  the  union  of  the  pronuclei  in  the  process  of  fertilisation, 
and  by  the  regular  distribution  of  equal  numbers  of  paternal 
and  maternal  chromosomes  to  the  daughter-nuclei  of  the 
dividing  egg-cell. 

As  a  matter  of  fact,  both  among  animals  and  plants,  the 
force  of  heredity  is  as  strong  on  the  father's  as  on  the  mother's 
side,  although  the  sperm-cell  often  contains  only  one- 
thousandth  or  one-hundred-thousandth  part  of  the  living 
protoplasm  contained  by  the  egg-cell.1  This  can  be  explained 
only  by  assuming  the  nuclei  of  the  two  germ-cells,  and  especially 
the  chromosomes  in  the  nuclei,  to  be  the  chief  material  bearers 
of  hereditary  properties.  Oskar  Hertwig  in  1898 2  pronounced 
this  to  be  his  opinion,  but  as  far  back  as  1884  he  and  Strasburger 
declared  the  nuclear  substance  to  be  what  Nageli  called  Idio- 
plasm. Boveri,  too,  says  very  aptly  on  this  subject : 3  '  However 
widely  the  male  and  female  germ-cells  may  differ,  they 
resemble  one  another  in  one  point,  viz.  their  nuclear  substance. 
The  full-grown  sperm-nucleus  is  indistinguishable  from  the 
egg-nucleus,  the  paternal  and  maternal  nuclear  elements  are 
absolutely  alike  in  size,  shape,  and  number.  All  imaginable 
care  is  shown  in  effecting  their  distribution  in  equal  proportions 
to  the  daughter-cells,  and,  as  we  may  assume,  to  all  the  cells 
of  the  embryo.4  In  these  paternal  and  maternal  nuclear 
elements  must  reside  the  directing  forces,  which  stamp  upon 
the  new  organism  not  only  the  characteristics  of  its  species, 
but  also  the  individual  qualities  of  both  parents  combined. 
This  combination  of  the  nuclear  elements  as  means  of  trans- 
mitting qualities  would  seem  to  be  the  object  of  all  copulation 
from  that  of  the  lowest  Infusorians  to  that  of  mankind.' 

In  our  task  of  considering  the  problems  of  fertilisation  and 

1  In  one  sea-urchin,  Toxopneustes,  the  bulk  of  the  spermatozoon  is  between 
TOWOTT  an(i  sWo^i  the  volume  of  the  ovum  (Wilson,  The  Cell,  p.  134). 

2  Die  Zetle  und  die  Gewebe,  II.  232,  &c. 

3  Das  Problem  der  Befruchtung,  p.  35.     Cf.  also  0.  Hertwig,  Allgemeine 
Biologic,  pp.  354,  &c. 

4  This  applies  especially  to  the  cells  in  the  germinal  tract  of  the  embryo. 
Some  deviation   from  this  law   may  occur  in  the  somatic  cells,  as  part  of 
the  chromatin  loops  is  thrown  off.     Cf.  pp.  123,  &c.,  and  p.  169. 


heredity,  we  have  here  arrived  at  one  important  result,  which 
we  can  regard  as  fairly  certain :  Fertilisation  consists 
essentially  in  the  nuclear  union  of  two  germ-cells,  and  through 
this  nuclear  union  the  parental  characteristics  are  transmitted 
to  the  offspring.  The  chromosomes  of  the  cell-nucleus  are 
shown  in  this  process  to  be  the  immediate  material  bearers  of 
heredity  in  the  organic  world. 

It  is  important  once  more  to  draw  attention  to  the  fact 
that,  in  the  nuclear  union  that  takes  place  in  fertilisation,  the 
chromosomes  of  the  two  pronuclei  retain  their  individuality. 
Whether — as  in  the  Echinus  -type  ! — the  male  and  female 
pronuclei  coalesce  and  form  one  common,  resting  cleavage- 
nucleus,  or  whether — as  in  the  Ascaris-type — the  two  pronuclei 
remain  distinct  until  they  break  up  in  forming  the  first  cleavage- 
spindle  of  the  fertilised  ovum  :  in  both  cases  alike  the  paternal 
and  maternal  chromosomes  remain  separate,  divide  themselves 
independently,  and  distribute  their  longitudinal  segments 
equally  between  the  two  daughter-nuclei  of  the  first  cleavage 
stage  of  the  ovum.  This  independent  action  on  the  part  of 
the  chroma  tin  derived  from  father  and  mother  respectively 
may,  as  V.  Haecker2has  shown,  be  traced  in  favourable  cases 
from  the  nucleus  of  the  fertilised  ovum,  through  numerous 
generations  of  cells,  to  the  nuclei  of  the  germ-cells  in  the 
embryo  resulting  from  this  fertilisation.  This  independence 
of  the  chromatin  elements  is  what  Boveri  calls  <  the  in- 
dividuality of  the  chromosomes  '  ;  to  some  extent  it  stamps 
these  morphological  constituents  of  the  cell  as  being  the 
visible  bearers  of  heredity. 

Boveri's  well-established  hypothesis  of  the  individuality  of 
the  chromosomes3  has  been  accepted  in  the  last  few  years, 

1  See  pp.  120  and  156  for  the  difference  between  these  two  types  of  fer- 

2  '  Uber  die  Autonomie  der  vaterlichen  und  mutterlichen  Kernsubstanz 
vom  Ei  bis  zu  den  Fortpflanzungszellen  '  (Anatomischer  Anzeiger,  XX,  1902). 
Rabl,  Boveri,  and  Riickert  have  made  similar  observations.     Cf.  Wilson,  The 
Cell,  p.  208  ;   O.  Hertwig,  Allgemeine  Biologic,  pp.  289,  &c. 

3  On  the  subject  of  Boveii's  theory  of  the  individuality  of  the  chromosomes, 
see  his  lecture  on  the  problem  of  fertilisation  (Das  Problem  der  Befruchtung, 
Jena,  1902),   and    also  the    following  works    by    the    same    author :   '  Uber 
mehrpolige  Mitosen  als  Mittel   zur  Analyse  des  Zellkerns  '    (Verhandl.   der 
physikal.-medizin.  Gesellschaft.,  Wiirzburg,  XXXV,  1902,  pp.  67-90);  '  Uber  die 
Konstitution    der    chromatischen    Kernsubstanz '    ( Verhandl.    der    Deutschen 
Zoolog.  Gesellsch.,  1903,  pp.  10-33);  '  Ergebnisse  iiber  die  Konstitution  der  chro- 
matischen Substanz  des  Zellkerns,'  Jena,  1904.    In  the  last-named  work  Boveri 


not  only  by  most  zoologists,  but  also  by  eminent  botanists, 
such  as  E.  Strasburger1  and  J.  Keinke;2  others,  however, 
such  as  Yves  Delage,  have  opposed  it,  whilst  it  has  been  only 
partially  adopted  by  E.  B.  Wilson  ('The  Cell,'  pp.  294-301) 3 
and  Oskar  Hertwig  ('  Allgemeine  Biologie,'  1906,  pp.  205-208). 
Should  it  be  fully  confirmed,  our  comprehension  of  the  material 
basis  of  heredity  would  undoubtedly  be  facilitated.  According 
to  Boveri,  the  chromosomes  during  karyokinesis  are  in  a 
state  of  rest,  and  in  this  condition  they  have  clearly  defined 
shapes  and  are  strongly  susceptible  to  nuclear  stains,  which 
render  them  visible  in  fixed  numbers.  When  the  fresh  nuclei 
of  the  daughter- cells  are  formed,  the  chromosomes  in  them 
revert  from  a  state  of  rest  to  one  of  activity,  in  which  they 
control  all  the  vital  functions  of  the  cell.  Their  free  ends 
approach  one  another,  unite  and  become  matted  together 
by  means  of  amoeboid  processes,  so  that  they  form  a  coil  of 
chromatin  thread  or  a  chromatin  network.  It  is  not  until 
the  next  division  of  the  cell  that  the  chromosomes  reappear 
in  the  same  form,  number,  and  order  as  before,  in  fact,  in  the 
same  '  individuality  ' ;  they  are  again  separate,  just  as  the 
oxygen  and  hydrogen  which  make  up  water  are  given  off 
again  when  the  water  is  resolved  into  its  chemical  constituents. 
In  the  case  of  chemical  compounds,  we  may  assume  persistence 
in  the  elements  of  which  they  are  composed,  and,  in  exactly 
the  same  way,  we  may  assume  a  similar  latent  persistence  of 

formulates  his  theory  most  precisely.  A  good  account  of  the  development  of 
the  theory  of  individuality  up  to  1900  is  given  by  Wilson,  The  Cell,  pp.  294-301. 
Fresh  confirmation  of  it,  in  a  department  where  it  was  formerly  contested,  is 
added  by  J.  Marechal,  '  t)ber  die  morphologische  Entwicklung  der  Chromo- 
somen  im  Keimblaschen  des  Selachiereis '  (Anatomischer  Anzeiger,  XXV,  1904, 
Nos.  16  and  17,  pp.  383-398)  and  '  Uber  die  morphologische  Entwicklung  der 
Chromosomen  im  Teleostierei '  (ibid.  XXVI,  1905,  No.  24,  pp.  641-652).  That 
the  chromosomes  are  not  to  be  regarded  literally  as  individuals  is  obvious, 
and  Boveri  himself  does  not  mean  this ;  he  considers  only  that  they  are 
clearly  denned  parts  of  the  cell,  capable  of  independent  division  and 

1  '  tiber  Reduktionsteilung  '  (Sitzungsber.  der  Berl.  Akad.  der  Wissensch., 
XIV,  1904,  pp.  587-614).     Cf.  p.  116. 

2  Philosophic  der  Botanik,  1905,  pp.  60,  69,  70,  143. 

a  Wilson  is  of  opinion  that  not  the  chromosomes  themselves,  but  only  their 
constituents,  the  chromomeres,  remain  as  constant  elements  through  all 
changes  in  the  nucleus.  ,  It  is,  therefore,  to  the  chromomeres  that  we  ought 
to  ascribe  '  individuality,'  rather  than,  as  Boveri  does,  to  the  chromo- 
somes. For  our  purpose  it  is,  however,  a  matter  of  indifference  whether  the 
chromosomes  or  the  chromomeres  should  eventually  be  proved  to  possess 


the  chromosomes  as  the  material  bearers  of  the  laws  of  organic 
development  during  the  whole  life  of  the  cell. 

On  p.  166  I  quoted  Boveri's  statement  to  the  effect  that 
in  the  cleavage-divisions  of  the  fertilised  ovum  the  chromo- 
somes of  the  cleavage-spindle  are  distributed  in  equal  propor- 
tions to  all  the  cells  of  the  embryo.  The  exceptional  cases, 
to  which  I  have  already  referred  shortly  (pp.  123,  &c.),  con- 
firm Boveri's  opinion  that  the  chromosomes  possess  a  certain 
individual  independence.  At  the  cleavage  of  the  ovum 
of  Ascaris  megalocephala  var.  bivalens,  from  the  two-cell  stage 
onwards,  the  four  chromosomes  of  the  cleavage-spindle  remain 
only  in  those  daughter- cells  which  are  to  supply  the  germ- 
cells  of  the  embryo,  whereas  they  undergo  a  striking  modifica- 
tion in  those  daughter-cells  which  are  to  produce  the  somatic 
cells.  In  these  the  ends  of  the  chromosomes  are  cast  off  and 
lost,  and  the  remaining  middle-piece  breaks  up  into  a  number 
of  little  rods  (see  p.  124,  fig.  23).  In  subsequent  divisions, 
giving  rise  to  somatic  cells,  the  chromosomes  always  appear 
in  this  form  and  order,  but  in  the  cells  of  the  germinal  area 
the  original  number,  form,  and  arrangement  of  the  chromo- 
somes are  preserved,  until  finally,  before  the  maturation- 
divisions  of  the  germ-cells,  the  ordinary  chromatin  reduction 
occurs,  and  the  number  of  chromosomes  is  reduced  to  half, 
and  then  is  brought  back  to  the  normal  by  fertilisation.  In 
biological  language  this  morphological  result  is  stated  thus : 
The  chromosomes  of  the  germinal  areas  represent  in  an 
unbroken  series  the  bearers  of  heredity  for  the  species  in 
question.  In  a  series  of  observations  made  in  1901  on  a 
water-beetle,  Dytiscus,  Giardina1  has  described  processes 
which  show  a  difference  in  the  nuclei  of  sexual  cells  and  somatic 
cells.  Here,  too,  in  the  former  chromatin  elements  remained 
constant,  which  were  lost  in  the  latter. 

Some  recently  discoveredjfacts  pointing  to  a  qualitative 
difference  in  the  chromosomes^of  one  and  the  same  nucleus  are 
very  significant.3  It  is  enough  for  the  present  to  say  that 
in  the  spermatogenesis  of  various  insects  (especially  in  bugs, 
beetles,  and  grasshoppers)  a  so-called  superfluous  or  accessory 

1  '  Origine  dell'  oocite  e  delle  cellule  nutrici  nel  Dytiscus  '  (Internat.  Monat- 
schr,  fur  Anatomic  und  Physiologic,  XVIII,  1901). 

2  Boveri  gives  a  short  summary  of  them.     '  Uber  die  Konstitution  der 
chromatischen  Kerasubstanz,'  pp.  20-26. 


chromosome  occurs,1  which  at  the  last  maturation- division 
passes  undivided  into  one  of  the  two  sperm-cells,  whilst  the 
other  receives  one  chromosome  less.  Montgomery  calls  these 
accessory  chromosomes  heterochromosomes  ;  he  has  observed 
them  in  the  spermatogenesis  of  spiders.2 

Button  noticed,  in  the  spermatogenesis  of  a  grasshopper — 
Brachystola  magna — (1900  and  1902),  that  in  the  secondary 
spermatogonia  (descendants  of  the  male  germ-cells)  not  only 
did  the  extra  chromosome  appear  regularly  for  nine  generations 
of  cells,  but  the  other  chromosomes  of  the  same  cells  fell  into 
two  groups  of  different  sizes,  and  always  occurred  in  pairs. 
Quite  recently  E.  B.  Wilson  has  made  a  very  careful  study  of 
the  qualitative  differences  of  the  chromosomes  in  the  germ- 
cells  of  bugs  (Hemiptera),  and  their  biological  functions.3 
He  distinguishes  normal  chromosomes,  or  idiochromosomes, 
from  abnormal  or  heterotropic  (accessory)  chromosomes. 
The  idiochromosomes  are  of  two  sizes,  which  he  calls  respec- 
tively macrochromosomes  and  microchromosomes ;  they 
occur  either  in  pairs  or  singly.  In  the  egg-cells  the  mature 
ova  invariably  contain  half  the  normal  number  of  chromo- 
somes, but  among  the  sperm-cells  there  are  three  different 
types,  with  chromosomes  varying  in  quality  or  quantity. 
Wilson  attempts  to  account  for  the  sex  differences  in  Hemiptera 
as  depending  upon  the  different  combinations  of  these  male 
chromosomes  with  the  female. 

When  we  consider  that  Mendel's  Law  of  Hybridisation,4 

1  For  a  fuller  account  of  it  see  Wilson,  The  Cell,  pp.  271,  272 ;  Korschelt 
and  Heider,   Vergleichende  Entwicklungsgesch.  der   wirbellosen  Tiere,  Allgem. 
Teil,     pp.     599-601  ;    B.     de    Sinety,    Recherches    sur    les    Phasmes,    1901, 
pp.  123-126  ;  Sutton,  '  The  Spermatogonial  Divisions  in  Brachystola  magna  ' 
(Kansas  Quarterly  Journal,  1900  and  1902) ;  J.  Pantel  and  R.  de  Sinety,  *  Les 
cellules  de  la  lignee  male  chez  le  Notonecta  glauca '  (La  Cellule,  XXIII,  1906, 
fasc.  I.  pp.  89-303,  138,  &c.,  245).     See  also  the  works  mentioned  on  p.  110, 
note  2. 

2  T.  H.  Montgomery,  '  Spermatogenesis  of  Syrbula  and  Lycosa,  with  general 
remarks   on   the   reduction   of    Chromosomes   and   on   Heterochromosomes ' 
(Proceedings   Acad.   Nat.   Science,   Philadelphia,   LVII,    1905,   pp.    161-205). 
Montgomery   classes   as  heterochromosomes   all   those   that   differ   from   the 
normal  in  size  or  structure.     Cf.  on  this  subject  a  review  in  the  Naturwissen- 
schaitliche  Rundschau,  1906,  No.  4,  p.  44. 

3  '  Studies   on   Chromosomes,'   I,   II,    and   III    (Journal  of  Experimental 
Zoology,  1905,  Nos.  3  and  4  ;  III,  1906,  No.  1). 

4  Greg  or  Mendel  (1822-84)  was  abbot  of  the  Augustinian  monastery  in 
Briinn.     Cf.  C.  Correns,  'Gr.  Mendels  BriefeanC.  Nageli,'  1867-73  (Abteil.  der 
mathemat.-physikal.  Klasse  der  Kgl.  Sdchsischen  Gesellsch.  der  W issenschaften 
XXIX,   3,   1905).     Mendel's  laws  of  segregation    are   dealt  with  very  fully 


which  is  to  a  great  extent  confirmed  by  the  phenomena  of 
hybrid  fertilisation,  may  have  a  quite^simple  morphological  basis, 
if  we  accept  Boveri's  theoryof  the  individuality  of  chromosomes 
(as  Boveri  himself  was  the  first  to  show),1  we  can  scarcely 
refrain  from  ascribing  to  the  chromosomes  a  certain  individual 
independence,  in  virtue  of  which  they  become  the  material 
bearers  of  heredity.  At  the  seventy- seventh  meeting  of 
German  naturalists  and  physicians  at  Meran  in  September  1905, 
the  interesting  connexion  existing  between  the  individuality 
of  the  chromosomes  and  Mendel's  Law  was  discussed  by 
C.  Correns  from  the  botanical  point  of  view,2  and  by  C.  Heider 
from  the  zoological  and  cytological.3  I  must  limit  myself 
here  to  a  very  brief  account  of  the  matter. 

Mendel's  Law  of  Hybridisation,  which  has  recently 
attracted  so  much  attention,  comprises  three  rules  :  the  rule 
of  dominance,  the  rule  of  segregation,  and  the  rule  of  independ- 
ence of  characters.  According  to  the  rule  of  dominance,  when 
two  sub-species  (e.g.  red  and  white  peas)  are  crossed,  the  hybrid 
offspring  of  the  first  generation  resemble  one  parent  (the  white 
pea)  in  every  respect,  and  the  characteristics  of  the  other 
parent  (the  red  pea)  do  not  show  themselves.  The  character 
that  appears  in  the  first  hybrid  generation  is  called  the  dominant, 
and  the  contrasted  character  that  disappears  is  called  the 
recessive.  According  to  the  rule  of  segregation,  if  the  breeding 
of  these  hybrids  be  continued,  the  contrasted  characters  of 
both  parents  are  again  distinguished  or  segregated,  and  in  such 
a  way  that  half  the  germ-cells  of  the  hybrid  tend  to  give 
rise  to  the  character  of  one  parent,  and  the  other  half  to  the 
character  of  the  other  parent.  According  to  the  rule  of 
independence  of  characters,  the  various  individual  characters, 

by  de  Vries,  in  the  second  volume  of  his  Mutationstheorie,  1903,  and  by  Lotsy 
in  his  Vorlesungen  uber  Deszendenztheorien,  1906,  Lecture  8.  They  have  been 
applied  to  cross-breeding  among  silkworms  by  K.  Toyama,  '  Mendel's  laws 
of  heredity  as  applied  to  the  silkworm  crosses'  (Biolog.  Zentralblatt,  XXVI, 
1906,  Nos.  11  and  12).  Cf.  also  J.  Gross,  '  Uber  einige  Beziehungen  zwischen 
Vererbung  und  Variation  '  (ibid.  Nos.  13-18).  According  to  Gross  (p.  414) 
no  typical  instances  of  Mendelism  occur  when  species  are  crossed. 

1  uber  die  Konstitution  der  chromat.  Kernsubstanz,  pp.  32-33.    J.  Reinke  too 
thinks  (Einleitung  in  die  theoretische  Biologie,  p.  539)  that  Mendel's  law  supports 
the  theory  that  the  chromosomes  are  the  chief  bearers  of  heredity. 

2  *  Uber    Vererbungsgesetze '    ( Verhandl.   der    77    Versammlung    deutscher 
Naturi.  und  Arzte,  Leipzig,  1906,  Part  I,  pp.  201-221). 

3  4  Vererbung  und  Chromosomen  '  (ibid.  pp.  222-244). 


which' [distinguished  the  parents  of  the  first  hybrid,  appear 
quite  ^independently  of  one  another,  when  cross-breeding  is 

When  two  sub-species  of  the  same  species  are  crossed,  and 
the  characters  of  the  offspring  follow  Mendel's  laws,  they  are 
said  to  '  mendelise.'  Mendel  formulated  his  law  in  consequence 
of  experimental  observations  on  hybridisation,  and  quite  apart 
from  microscopic  research.  Working,  however,  on  other  lines, 
cytologists  have  found  three  important  principles,  which  lead 
them  to  regard  the  chromosomes  as  the  material  bearers  of 
heredity  and  to  ascribe  to  them  a  certain  individual  independ- 
ence. Firstly,  each  germ-cell  receives  exactly  half  the  normal 
number  of  chromosomes,  and  of  those  which  it  contains,  half 
are  paternal  and  half  maternal.  Secondly,  each  germ-cell 
receives  the  total  number  of  chromosomes  necessary  to  normal 
development,  these  chromosomes  being  parental  in  origin,  but 
qualitatively  different.  Thirdly,  these  chromosomes  may 
meet  in  the  germ-cells  of  the  offspring  in  very  various  com- 
binations (arranged  mostly  in  tetrads  or  groups  of  four),  and 
there  they  form  regularly  fresh  combinations  in  their  matura- 
tion-divisions and  fertilisation. 

If  we  may  assume  that  qualitatively  different  chromosomes 
are  the  bearers  of  definite  hereditary  qualities,  these  three 
principles  will  enable  us  easily  to  explain,  not  only  Mendel's 
three  rules,  but  also  most  of  the  other  phenomena  of  variation 
and  heredity. 

Of  the  numerous  instances  quoted  by  Correns  and  Heider 
in  the  above-mentioned  lectures,  I  may  give  one  by  way  of 

A  red  and  a  white  specimen  of  Mirdbilis  Jalapa  were 
crossed.  The  hybrids  of  the  first  generation  all  bore  pink 
blossoms,1  those  of  the  second  generation  were  partly  white, 
partly  red,  partly  pink,  in  the  ratio  1:1:2;  so  that  the 
pink  blossoms  were  twice  as  numerous  as  either  the  white  or 
the  red.  Let  us  assume  (see  Plate  II)  that  the  tendency  to 
produce  red  blossoms  is  represented  by  a  definite  chromosome 

1  According  to  Mendel's  rule  of  dominance  the  red  colour  of  one  parent 
ought  to  have  been  the  dominant,  but  this  was  not  the  case.  The  rule  of 
dominance,  therefore,  is  not  illustrated  by  this  example,  and  it  is  more  difficult 
to  account  for  it  by  the  chromosome  theory  than  for  the  rule  of  segregation. 
On  this  subject  see  p.  173,  note  2,  of  Gross's  work. 


A,  and  the  tendency  to  produce  white  blossoms  by  a  definite 
and  qualitatively  different  chromosome  a.  The  red  variety 
of  Mirabilis  Jalapa  has  among  its  chromosomes  only  A,  the 
white  variety  only  a,  as  influencing  the  colour  of  the  blossom. 
The  first  hybrid  generation  receives  in  its  fertilised  egg-cell 
and  in  all  the  somatic  cells  the  combination  A-|-a,  i.e.  all  its 
blossoms  are  pink.  At  the  maturation-divisions  of  the  germ- 
cells  of  this  first  hybrid  generation  a  separation  of  the  A  -|-  a 
pair  of  chromosomes  takes  place,  and  by  the  reduction  processes 
half  of  all  the  mature  germ-cells  receive  chromosome  A,  and 
the  other  half  chromosome  a.  What  is  the  result  to  the 
second  generation,  produced  by  the  union  of  these  germ-cells 
in  twos  ?  In  the  somatic  cells  the  chromosomes  will  be  thus 
combined,  A-f-A,  A+a,  a+A,  a-\-a,  and  each  combination 
will  probably  occur  the  same  number  of  times  ;  in  other  words, 
in  this  generation  there  will  be  pink  blossoms  as  well  as  pure 
red  and  pure  white,but  the  pink  will  be  about  twice  as  numerous, 
which  was  actually  found  to  be  the  case.  Plate  II  at  the  end 
of  the  book  illustrates  this  relation  of  the  chromosome  theory 
to  the  phenomena  of  hybridisation.  The  diagrams  were  used 
in  Heider's  lecture. 

We  have  now  learnt  to  regard  the  mixture  of  qualities  as  the 
chief  aim  of  fertilisation,  in  which  the  combined  properties  of 
both  parents  are  transmitted  to  their  offspring,  and  we  have 
seen  further  that  the  chief  part  in  this  transmission  is  played 
by  the  chromosomes  of  the  cell-nucleus.  The  next  question  we 
must  answer  is  this  :  What  is  the  object  of  this  blending  of 
qualities  ?  Why  is  it  of  so  much  importance  to  the  main- 
tenance of  organic  species  that  Nature  has  taken  great 
pains  to  secure  it,  by  means  of  these  complicated  and  regular 
arrangements  ? 

The  opinions  held  on  this  subject  are  to  some  extent  con- 
tradictory. We  may  safely  take  it  for  granted  that  the 
rejuvenating  or  regenerating  effect,  ascribed  by  Biitschli, 
K.  Hertwig,  A.  Biihler  and  others  to  the  process  of  fertilisation, 
is  due,  at  least  in  part,  to  this  blending  of  qualities.  But  I 
have  already  referred  to  their  theories  (pp.  162,  &c.),  and  so  we 
need  now  only  answer  the  question  :  What  is  the  significance 
of  blending  qualities  for  the  race  development  of  different 
species  ?  Does  it  act  in  a  conservative  or  in  a  liberal  sense  ? 


Does  it  promote  permanence  of  species  or  does  it  supply  the 
means  of  altering  them  ? l 

Charles  Darwin,  Spencer,  Romanes,  Hatschek,  0.  Hertwig 
and  others  have  regarded  this  blending  of  parental  qualities 
effected  by  fertilisation  as  a  means  of  compensating  for 
individual  fluctuations  ;  they  are  therefore  of  opinion  that 
this  union  of  qualities  preserves  the  purity  of  the  race,  and  so 
makes  for  permanence. 

According  to  these  authors  it  would  be  possible  for  a  new 
variety,  race,  or  species  to  arise  only  if  the  possibility  of  breeding 
with  individuals  of  the  same  species  were  restricted,  by  either 
exterior  or  interior  circumstances,  to  definite  and  limited 
groups  of  individuals,  which  then  had  the  power  to  propagate 
and  intensify  their  peculiarities.  On  this  idea  are  based 
Wagner's  theory  of  migration,  Romanes'  theory  of  physiological 
selection,  Gulick's  theory  of  segregation,  &c. 

August  Weismann's  view  is,  however,  directly  opposed  to  all 
these.3  He  thinks  that  amphimixis,  i.e.  the  mixing  of  qualities 
resulting  from  fertilisation,  is  the  chief  means  of  modifying 
species.  It  gives  rise  to  fresh  combinations  of  the  nuclear 
elements,  and  to  corresponding  new  variations  in  the  hereditary 
qualities  of  the  offspring.  These  variations  offer  a  wide  field 
for  natural  selection,  which  '  breeds  '  from  them  new  races  and 

At  first  sight  this  theory  is  very  attractive.  Let  us  assume 
that  the  male  and  female  pronuclei  of  the  germ-cells  of  some 
organic  species  possess  each  eight  chromosomes  before  their 
union  in  the  process  of  fertilisation,  and  that  these  sixteen 
chromosomes  differ  qualitatively  from  one  another.  In  the 
cleavage-spindle  of  the  fertilised  ovum  they  may  be  paired  in 
no  less  than  sixty-four  different  ways,  and  so  may  produce 
sixty-four  descendants,  all  differing  qualitatively  from  one 
another  and  from  their  parents.  Now,  as  a  matter  of  fact,  in 
most  species  of  plants  and  animals  the  number  of  chromosomes 
is  far  higher  than  sixteen,3  and  therefore  the  possible  number 
of  variations  due  to  fertilisation  is  correspondingly  higher. 
It  appears  to  be  true  that  by  blending  qualities  a  very  vast 

1  See  Korschelt  and  Heider,  Lehrbuch  der  vergl.  Entwicklungsgesch.,  pp.  702,  &c. 

2  Lectures  on  the  Evolution  Theory,  I,  pp.  331,  &c.;  II,  pp.  192-237  (Eng.  trans.). 

3  See  Chapter  V.  pp.  92  and  93. 


field  is  opened  to  natural  selection.  Boveri  agrees  with 
Weismann  to  a  certain  extent,1  and  thinks  that  the  mixture 
of  qualities,  which  is  the  chief  object  of  fertilisation,  is  one 
means,  and  even  one  of  the  most  efficacious  means,  whereby 
organic  species  have  developed  from  the  simplest  Protozoa  to 
the  highest  animals  and  plants. 

My  own  opinion  nevertheless  is,  that  the  amphimixis 
resulting  from  fertilisation  may  not  be  of  such  importance  to 
the  evolution  theory  as  Weismann  believes.3  I  need  not  now 
lay  much  stress  on  the  many  objections  to  it  that  can  be 
raised.  For  instance,  it  is  quite  common  to  find  the  number 
of  chromosomes  differing  greatly  in  closely  connected  species 
of  animals  and  plants — e.g.  in  the  Ascaris  class  of  worms — 
whilst  forms  as  far  removed  from  one  another  as  the  frog, 
the  salamander,  the  mouse,  the  salmon,  a  crab  (Branchipus), 
a  bug  (Pyrrhocoris)  and  the  lily  all  have  the  same  number 
of  chromosomes,  viz.  twenty-four.  Some  experimental 
evidence  is  needed  to  show  that  the  variability  of  the 
species  is  directly  connected  with,  and  dependent  upon, 
the  number  of  its  chromosomes.  Weismann  anticipated 
these  difficulties  by  suggesting,  in  his  theory  of  determinants, 
that  only  the  larger  complexes  of  bearers  of  heredity  (the  ids) 
correspond  to  the  chromosomes  ;  each  of  these  is  built  up  of  a 
great  number  of  smaller  bearers  of  heredity  (determinants), 
which  are  equivalent  to  the  chromomeres  or  smallest  grains 
of  chromatin  in  the  chromosomes,  and  are  able  to  vary  in- 
dependently of  one  another.  As  very  little  is  actually  known 
of  the  finer  structure  of  the  chromosomes,3  these  theoretical 
speculations  cannot  be  tested  by  means  of  microscopical 

There  are,  however,  other  objections  to  Weismann'a  theory 
of  the  importance  of  amphimixis,  and  they  are,  perhaps,  of 
greater  weight.  We  must  notice  at  the  outset  that  indiscri- 
minate cross-breeding  between  individuals  of  the  same  species 

1  Das  Problem  der  Befruchtung,  pp.  36-38. 

2  We  must  be  careful  to  distinguish  amphimixis  in  Weismann's  sense, 
in  which  it  refers  to  the  blending  of  qualities  of  individuals  belonging  to  the 
same  species,  from  the  other  use  of  the  word,  in  which  it  refers  to  sexual  cross- 
breeding between  individuals  of  different  species.     I  shall  discuss  the  latter 
kind  of  amphimixis,  as  bearing  upon  the  Evolution  theory,  in  Chapter  IX, 
'  Theory  of  permanence  or  theory  of  descent.' 

3  Wilson,  The  Cell,  pp.  301,  302. 


can  never  lead  to  a  new  permanent  variety,  ag  the  average 
will  always  recur.  Moreover,  a  completely  new  quality  in  the 
offspring  can  never  be  produced  by  a  mere  combination  of 
qualities  present  in  the  parents.  It  is  therefore  difficult  to  see 
how  a  mixture  of  qualities  can  ever  give  rise  to  new  species, 
families,  classes,  &c.,  in  which  some  new  organ  or  system  of 
organs  is  frequently  the  distinguishing  characteristic.  Natural 
selection  is,  according  to  Weismann,  the  sole  directive  element 
in  the  evolution  of  a  race,  but  all  that  it  can  do  is  to  make 
choice  out  of  the  variations  furnished  by  amphimixis,  and  to 
preserve  the  individuals  best  capable  of  existence,  and  therefore 
Weismann's  whole  theory  of  evolution  seems  unsatisfactory  ; 
mere  amphimixis  and  selection  could  never  have  produced 
the  present  system  of  animals  and  plants  from  extremely 
simple  primitive  organisms. 

Since  1895  Weismann  has  very  ingeniously  tried  to  meet 
this  objection  by  bringing  forward  his  theory  of  germinal  selec- 
tion as  a  new  factor  in  evolution.  He  now  no  longer  regards 
the  determinants  of  hereditary  qualities  in  the  nuclear  substance 
of  the  germ-cells  as  invariable,  but  is  of  opinion  that  they 
1  are  continually  oscillating  hither  and  thither  in  response 
to  very  minute  nutritive  changes,  and  are  readily  compelled  to 
variation  in  a  definite  direction,  which  may  ultimately  lead  to 
considerable  variations  in  the  structure  of  the  species,  if  they 
are  favoured  by  personal  selection,  or  at  least  if  they  are  not 
suppressed  by  it  as  prejudicial.'  l 

He  goes  so  far  as  to  speak  of '  vital  affinities/ 2  i.e.  of  definite 
interior  forces  uniting  the  determinants  into  ids,  and  the 
biophors  into  determinants.  It  is  undoubtedly  a  very  interest- 
ing concession  on  Weismann's  part,  when  he  says  :  3  'In  all 
vital  units  there  are  forces  at  work  which  we  do  not  yet  know 
clearly,  which  bind  the  parts  of  each  unit  to  one  another  in  a 
particular  order  and  relation.'  Weismann  seems  here  to 
acknowledge  that  it  is  impossible  ever  to  understand  a  develop- 
ment of  the  organic  world,  with  definite  arrangement,  and 
consequently  ordered  in  conformity  to  law,  unless  there  are 
interior  laws  governing  that  development.  If — as  Weismann 

1  Weisinann,  Evolution  Theory,  II,  p.  196,  Eng.  trans. 

2  Ibid.  I,  p.  374  ;  II,  p.  36. 

3  Ibid.  II,  p.  35. 


suggests  in  these  quotations — there  is  a  connexion,  tending 
to  some  aim,  between  the  material  bearers  of  heredity  among 
themselves  and  the  influences  of  the  outer  world,  so  that  the 
former  are  modified  by  the  latter  and  directed  into  new  channels 
of  development,  he  seems  also  to  grant  that  there  is  a_teleological 
element  in  the  constitution  of  these  material  bearers  of  heredity, 
to  which  they  own  their  capacity  to  adapt  themselves  to  new 
circumstances  by  corresponding  changes  in  their  constitution, 
and  thereby  to  effect  a  regular  development  of  the  organic 

This  teleological  element,  which  I  have  described  as  the 
interior  laws  governing  the  development  of  organisms,  is  no 
1  mystical,  intangible  thing  '  hovering  vaguely  in  the  air,  as 
some  of  my  opponents  have  imagined.  It  is  the  original 
chemico-physical  and  morphological  constitution  belonging  to 
the  first  bearers  of  the  hereditary  qualities  of  the  race,  at  least 
in  its  material  aspect.  If  we  wish  to  explain  the  phenomena 
of  heredity,  we  must  consider  in  this  material  constitution 
not  only  the  morphological  character  of  the  smallest  and  most 
elementary  parts  of  living  substance,  that  make  transmission 
of  qualities  possible,  but  also  their  dynamic  and  physiological 

It  cannot  be  denied  that  we  need  moreover  some  formal 
principle  to  explain  these  laws  of  evolution.  J.  Keinke,  the 
well-known  botanist,  has  lately  acknowledged  this,  by  declaring 
the  chromosomes  of  the  nucleus  to  be  the  chief  agents-,  in  all 
probability,  in  transmitting  specific  dominants.3 

Hans  Driesch,3  one  of  the  best  and  most  thorough  students 
of  organic  development,  seems  to  hold  a  very  similar  opinion, 
for  he  says  that  the  processes  of  organic  development  require, 

1  On  this  subject  see  J.  Reinke,  Philosophic  der  Botanik,  Leipzig,   1905, 
p.  106  ;  0.  Hertwig,  Allgemeine  Biologic,  1906,  chapter  xii,  '  Die  Physiologie 
des  Befruchtungsprozesses.'      From  what  is  stated  above  and  also  from  what 
follows,  it  is  plain  that  Gemelli,  in  his  Italian  translation  of  the  last  edition  of 
this  work  (Wasmann-Gemelli,  La  Biologia  moderna,  1906,  pp.  218-221),  com- 
pletely misunderstands  me,  if  he  thinks  that  I  regard  the  chromosomes  as  a 
transmitting  substance  in  a  purely  morphological  sense. 

2  Cf.  J.  Reinke,  Einkitung  in  die  theoretische  Biologic,  1901,  p.  455  ;  see  also 
pp.  386-408  and  especially  p.  396.     Cf.  further  his  Philosophic  der  Botanik, 
1905,  pp.  53,  &c.,  pp.  71,  &c. 

3  Of  his  works  cf.  especially  the  following  :  Die  organischen  Eegulationen  ; 
Vorbereitungen    zu   einer  Theoric   des  Lebens,  Leipzig,   1901  ;     Die  Seele  als 
elementarer     Naturfaktor,     Leipzig,      1903 ;      Kritisches      und      Polemisches 
(Biolog.  Zentralblatt,  1902,  Nos.  5,  6,  14,  15  ;   1903,  Nos.  21,  22,  23). 


as  an  indispensable  directive  power,  a  teleological  formal 
principle  which  may  be  compared  with  the  entelechies.  If 
this  is  true  for  the  development  of  the  individual,  we  may 
regard  it  as  still  more  necessary  for  the  hypothetical  develop- 
ment of  the  race.  I  shall  recur  to  this  topic  at  the  close  of 
Chapter  VIII  ( '  The  Problem  of  Life  ' ),  and  in  the  course 
of  Chapter  IX  ( '  Thoughts  on  the  Theory  of  Evolution  '). 

From  the  evidence  given  in  the  present  chapter  it  appears 
that  we  may,  with  great  probability,  regard  the  chromosomes 
of  the  nuclei  in  the  germ-cells  as  the  chief  material  bearers  of 

We  have  now  obtained  a  scientific  foundation  for  the 
interior  laws  of  development,  which  are  the  necessary  premiss 
for  the  hypothesis  of  a  race  evolution  of  organic  species.  I 
shall  have  to  deal  with  this  hypothesis  in  a  subsequent  chapter  : 
'  Thoughts  on  the  Theory  of  Evolution.'  For  the  present  I  will 
only  draw  the  reader's  attention  to  the  fact  that  all  the  results 
of  modern  biological  research,  in  this  department  as  in  others, 
increase  our  appreciation  of  the  Creator's  wisdom  and  power, 
and  show  us  in  what  a  simple  and  yet  wonderfully  regular  way 
the  transmission  of  the  parents'  qualities  to  their  descendants 
is  effected,  by  means  of  most  diminutive  material  portions  of 
the  germ  substance. 

1  Further  information  of  great  interest,  and  tending  to  confirm  this  theory, 
may  be  found  in  C.  Correns'  lecture  Uber  Vererbung  (On  Heredity)  and 
C.  Heider's  Vererbung  und  Chromo&omen  (Heredity  and  Chromosomes). 
These  lectures,  to  which  I  have  referred  on  p.  171,  were  delivered  in  September 
1905,  at  the  seventy-seventh  meeting  of  German  naturalists  at  Meran. 




There  are  no  organisms  more  simple  in  construction  than  the  cell  (p.  180). 
Bathybius  (p.  181).  Monera  (p.  181).  Absence  of  nucleus  in 
Bacteria  (p.  182).  Non-nucleate  red  blood-corpuscles  (p.  185).  Free 
nuclear  formation  (p.  186).  The  cell  not  composed  of  lower 
elementary  units  (p.  187).  The  idea  of  individuality  in  unicellular 
and  multicellular  creatures  (p.  188).  Energids  (p.  189).  Survey 
and  criticism  of  the  hypothetical  living  units  of  the  lowest  rank 
(p.  190). 


What  is  spontaneous  generation  ?  (p.  193).  Untenable  character  of 
the  chemico-physical  theories  of  spontaneous  generation  (p.  195). 
Radium  and  spontaneous  generation  (p.  197).  Untenable  character 
of  the  biological  theories  of  spontaneous  generation  (p.  198). 
History  of  the  theory  (p.  199).  Gradual  refutation  of  the 
theory  by  modern  biology  (p.  201).  Theory  of  spontaneous  genera- 
tion not  a  postulate  of  science  (p.  204).  Theory  of  creation  a  true 
postulate  of  science  (p.  206). 

I  HAVE  already  shown  (Chapter  III,  pp.  55,  65,  &c.)  that  the 
cell  is  not  a  simple  entity,  but  a  compound  formation  of  very 
delicate  and  artistic  structure,  as  recent  research  has  proved. 
We  have  also  considered  the  life  of  the  cell  (Chapter  IV)  and 
convinced  ourselves  of  the  great  and  universal  importance 
of  the  nucleus  in  every  function  of  cellular  life,  but  especially 
in  cell-division  and  in  the  processes  of  fertilisation  (Chapters 
V  and  VI).  We  have  now  sufficient  material  at  our  disposal  to 
enable  us  to  answer  with  assurance  the  question  propounded 
long  ago  :  '  Is  the  cell  the  ultimate  unit  of  organic  life,  or  is  it 
merely  an  aggregation  of  still  more  elementary  units  ?  '  The 
solution  of  this  problem  will  help  us  to  form  a  really  scientific 
opinion  on  spontaneous  generation  or  generatio  aequivoca,  for 
almost  all  attempts  to  disprove  the  unity  of  the  cell  have  been 
motived  by  a  desire  to  make  the  origin  of  organic  life  in  the 
world  more  intelligible  by  the  assumption  of  spontaneous 

179  N2 



The  question  of  the  unity  of  the  cell  resolves  itself  into  two 
other  questions,  which  we  shall  answer  each  in  turn.  The 
first  is  :  *  Are  there  really  in  nature  organic  entities  of  a  still 
lower  organisation  than  the  cell  ?  '  The  second  is  :  'Do  the 
morphologically  different  elements  of  the  cell  form  together 
one  biologically  indivisible  unit,  or  can  they  be  divided  into 
subordinate  biological  units  ?  '  On  the  answers  which  facts 
supply  to  these  questions,  depends  our  acceptance  of  the 
various  theories  which  represent  the  cell  as  a  mere  aggregation 
of  lower  units,  or  our  rejection  of  the  same  as  fictions.  What 
does  recent  research  tell  us  as  to  the  existence  of  living  entities 
of  still  lower  organisation  than  the  cell  ?  It  has  really  answered 
this  question  plainly  enough  already  ;  it  has  shown  us  that 
the  cell-nucleus  is  also  the  principle  of  organisation  for  the 
living  cell,  directing  its  most  important  vital  activities,  and, 
by  means  of  heredity,  maintaining  the  continuity  of  organic 
life.  Consequently  we  should  expect  to  find  no  organism  with 
a  protoplasmic  body  containing  no  nucleus,  and  none  with  a 
nucleus  that  is  not  inserted,  or  meant  to  be  inserted,  in  a  proto- 
plasmic body. 

This  does  not,  however,  prove  that  in  all  organisms  the 
cell-nucleus  must  be  developed  in  equal  perfection.  On  the 
contrary,  the  graduated  perfection  of  organic  beings  may 
extend  also  to  the  organisation  of  the  cells,  and  we  need  not 
be  surprised  to  find,  even  among  the  lowest  living  creatures, 
some  in  which  the  nucleus  is  not  formed  into  one  morphological 
whole,  but  is  scattered  in  little  grains  of  chromatin  (chromidia) 
about  the  protoplasm  of  the  cell.  As  we  shall  see  directly, 
this  occurs,  apparently  at  least,  in  many  Bacteria.  In  Chapter 
III,  p.  49,  I  pointed  out  that  the  nucleus  was  essential  to 
the  existence  of  the  cell,  either  in  a  complete  and  centralised 
form,  or  in  a  diffused  and  incomplete  one.  This  latter  state- 
ment need  not  surprise  us,  as  we  have  seen,  in  Chapters  V  and 
VI,  that  during  indirect  cell- division  the  distinct  nucleus 
ceases  for  a  time  to  exist  as  such,  because  the  nuclear  membrane 
breaks  up  and  the  chromatin  framework  of  the  nucleus  divides 
into  small  pieces,  viz.  the  chromosomes,  and  is  only  reorganised 


in  the  newly  formed  nuclei  of  the  daughter-cells.  The  sharply 
denned  form  of  the  nucleus  is  not  therefore  essential  to  the 
cell,  although  the  presence  of  the  nuclear  substance  is  essential. 

Attempts  have  been  made  to  demonstrate  the  existence  of 
really  non-nucleate  primitive  organisms,  or  at  least  to  assert 
the  possibility  of  their  existence.  Let  us  examine  them  in 
order  and  test  their  value. 

For  a  short  time  it  was  believed  that  the  long- sought 
organic  matter,  devoid  of  all  structure,  which  Ernst  Haeckel 
announced  as  the  Promised  Land  of  Darwinism,  had  really 
been  discovered.  The  discovery  was  made  whilst  the  North 
Atlantic  cable  was  being  laid  in  1857.  Huxley  subsequently 
described  this  primitive  matter  as  consisting  of  little  organic 
masses,  without  nucleus  and  without  any  structure,  found 
at  the  bottom  of  the  ocean,  and  named  by  him  Batkybi'&s 
Haeckelii,  after  the  famous  prophet  of  Darwinism.  But  the 
godfather  himself  was  obliged  later  on  to  declare  this  hopeful 
scion  of  the  Evolution  Theorj^  to  be  a  changeling,  foisted  upon 
him  by  an  impish  trick  of  bad  luck.  He  had  to  withdraw  his 
discovery,  and  acknowledge  that  there  had  been  a  mistake, 
about  the  Bathybius.  It  was  nothing  but  a  deposit  formed 
accidentally  in  a  test-tube  filled  with  alcohol.  Bessels,  the 
explorer  of  the  North  Pole,  afterwards  thought  that  he  had 
rediscovered  the  primitive  organism,  which  he  called  Proto- 
bathybius  ;  but  in  spite  of  the  amoeboid  movements  that  he 
said  he  observed,  the  ProtobatJiybius^  has  not  yet  been  admitted 
to  the  rank  of  a  living  creature  ;  at  best  it  appears  to  be  a 
deposit  of  organic  substance  which  has  formed  at  the  bottom 
of  the  sea  from  the  remains  of  plancton  organisms.  Haeckel's 
own  creations,  the  ostensibly  non-nucleated  Monera,  still  de- 
mand consideration.  Haeckel  classed  together  as  Monera, 
the  lowest  division  of  Protozoa,  all  those  that  he  thought 
contained  no  nucleus.  Their  number  seemed  at  first  to  be 
legion,  and  to  justify  the  hopes  set  upon  them  by  the  advocates 
of  the  Evolution  Theory.  But  as  our  microscopes  and  our 
methods  of  research  were  improved,  they  melted  away  like 
snowflakes  in  the  sunshine.  Apochromatic  objectives  and 
modern  staining  methods  have  revealed  the  hitherto  obscure 
nucleus  in  almost  all  Protozoa,  and  all  possessors  of  a  nucleus 
were  at  once  banished  from  the  class  of  Monera,  which  grew 


smaller  and  smaller.  The  day  is  not  far  distant  when  the  last 
Moneron  will  share  the  fate  of  the  last  of  the  Mohicans.  On 
this  subject  we  may  refer  to  R.  Hertwig,  an  eminent  zoologist 
and  a  favourite  pupil  of  Haeckel's.  In  the  seventh  edition  of 
his  '  Lehrbuch  der  Zoologie  '(1905,  p.  159),  he  writes  as  follows  : 
'  The  most  important  feature  in  the  Monera  is  said  to  be  the 
lack  of  a  nucleus.  Like  every  negative  characteristic,  this  is 
somewhat  unsatisfactory.  In  many  cases  it  is  difficult  to 
recognise  nuclei,  especially  when  the  protoplasm  is  abundant 
and  filled  with  chromatin  granules,  and  thus  it  may  happen 
that  animals  are  described  as  devoid  of  nucleus,  simply  because 
the  existing  nucleus  has  been  overlooked.  For  this  reason  the 
number  of  "  Monera  "  was  at  one  time  very  large  ;  it  has 
diminished,  as  improved  technical  methods  have  revealed 
nuclei,  and  so  it  is  not  only  possible,  but  even  probable,  that, 
in  the  few  forms  still  reckoned  as  Monera,  the  nuclei  have  only 
escaped  notice  ;  perhaps  their  functions  are  discharged  by 

Unicellular  animals  without  a  nucleus  have  therefore  no 
longer  any  scientific  justification  for  existence  ;  and  no  one 
can  refer  to  them  as  affording  evidence  of  there  being  living 
creatures  of  a  still  lower  degree  of  organisation  than  cells 
possess.  It  may,  however,  be  asked  :  Can  the  long-sought 
non-nucleated  forms  be  discovered  amongst  the  lowest  plants  ? 

Botanists  are  still  not  agreed  as  to  the  presence  of  a  genuine 
cell-nucleus  in  Bacteria  and  Cyanophyceae,  to  which  the 
Oscillaria  also  belong.1 

Biitschli  thought  that  he  had  discovered  in  Bacteria  a  very 
large  nucleus,  not  clearly  marked  off  from  the  layer  of  cyto- 
plasm, but  Fischer  contradicted  this  statement.  Arthur  Meyer 
('  Flora,'  1899,  pp.  428,  &c.)  believed  that  several  little  nuclei 
could  be  traced  in  the  cells  of  some  Bacteria.  Fritz  Schaudinn 

1  For  the  bibliography  of  this  subject,  see  Strasburger,  Lehrbuch  der  Botanik, 
sixth  edition,  1904  ;  Biitschli,  Weitere  Ausfuhrungen  uber  den  Ban  der  Cyano- 
phyceen  und  Bakterien,  Leipzig,  1896 ;  Fischer,  Untersuchungen  uber 
den  Ban  der  Cyanophyceen  und  Bakterien,  Jena,  1897  ;  G.  Schlater,  '  Zur 
Biologic  der  Bakterien:  Was  sind  Bakterien?'  (Biolog.  Zentralblatt,  .  1897, 
pp.  833,  &c.)  ;  J.  Reinke,  Einleitung  in  die  theoretische  Biologie,  Berlin,  1901, 
chapter  25,  pp.  256,  &c.  ;  R.  Hertwig,  '  Die  Protozoen  und  die  Zellentheorie,' 
Archiv  fur  Protistenkunde,  I,  1902,  pp.  1-40)  ;  Fr.  Schaudinn,  '  Beitrage  zur 
Kenntnis  der  Bakterien  und  verwandter  Organismen  '  (Archiv  fiir  Protis- 
tenJcunde,  I,  1902,  pp.  306,  &c.  ;  II,  1903,  pp.  421,  &c.). 


has  discovered  quite  recently  that  in  the  case  of  Bacillus 
Butschlii,  a  large  parasitical  fission  fungus  found  in  the  intestine 
of  the  cockroach,  Periplaneta  orientalis,  a  genuine  nucleus 
appears  temporarily  during  the  formation  of  spores,  although 
otherwise  the  nuclear  substance  is  dispersed  in  the  cell.  K. 
Hertwig's  investigations  into  Bacteria  and  Oscillaria  have 
led  him  to  conclude  that  these  organisms  ought  to  be  regarded 
as  cells  without  a  clearly  differentiated  nucleus,  but  having 
the  nuclear  substance  distributed  among  the  protoplasm. 
He  gives  the  name  chromidia  to  the  little  particles  of  chromatin 
in  Bacteria,  corresponding  to  the  chromosomes  and  their 
constituents,  the  chromomeres,  in  true  nuclei. 

J.  Keinke  does  not  venture  to  express  a  general  opinion 
as  to  the  non-nucleate  character  of  Cyanophyceae  and  Bacteria, 
but  he  considers  that  the  cell  of  Beggiatoa,  a  tiny,  thread- 
like Bacterium,  is  non-nucleate  to  this  extent,  that  it  does 
not  contain  any  distinct  nucleus,  in  the  sense  in  which  the 
higher  plants  and  animals  contain  nuclei. 

In  the  sixth  edition  of  his  *  Lehrbuch  der  Botanik,'  p.  46, 
Strasburger  says  :  '  The  two  most  essential  constituents  of 
the  protoplasm  (i.e.  of  the  living  cell)  are  the  nucleus  and  the 
cytoplasm,  and  the  vital  functions  of  the  cell  depend  upon 
the  interaction  between  them.  But  in  the  lowest  plants, 
Cyanophyceae  and  Bacteria,  the  existence  of  a  nucleus  is  still 
uncertain.'  On  p.  270  of  the  same  book,  Schenk,  in  writing 
of  Bacteria,  remarks  :  '  In  the  protoplast  there  are  one  or 
more  granular  structures  called  chromatin-bodies,  which 
may  be  deeply  coloured  by  stains,  and  are  regarded  as  nuclei 
by  various  authors.  Hitherto  no  one  has  succeeded  in 
demonstrating  undoubted  karyokinesis  in  them,  and  therefore 
the  presence  of  nuclei  (in  Bacteria)  is  still  not  established.' 
On  p.  274  Schenk  remarks  with  reference  to  the  Cyanophyceae  : 
'  Within  the  coloured  zone  (of  the  protoplast)  lies  the  colourless 
central  body,  which  perhaps  corresponds  to  a  nucleus.  How- 
ever, the  structure  and  division-figures  characterising  typical 
nuclei  have  not  been  observed  with  any  degree  of  certainty.' 

F.  G.  Kohl  on  the  other  hand,  in  a  recently  published 
work,1  declares  with  assurance  that  the  central  body  in  the 

1  *  t)ber  die  Organisation  und  die  Physiologic  der  Cyanophyceenzelle  und 
die  mitotische  Teilung  ihres  Kerns  '  (mit  10  Tafeln),  Jena,  1903. 


Cyanophyceae  is  a  true  nucleus,  and  he  proves  such  to  be  the 
case  from  the  processes  of  mitotic  division  that  occur.  Orville 
P.  Phillips l  has  come  to  the  same  conclusion,  and  thinks  that 
the  Cyanophyceae  can  no  longer  be  regarded  as  devoid  of 

The  existence  of  true  nuclei  in  Bacteria  has  lately  been 
asserted  also  by  E.  Eaymann  and  E.  Kruis,2  and  by  F. 

Even  if  we  are  obliged  to  regard  the  question  of  the 
non-nucleate  character  of  Bacteria  and  other  diminutive 
representatives  of  the  lowest  vegetable  orders  as  to  some 
extent  still  doubtful,  we  can  at  least  learn  from  the  investi- 
gations made  on  the  subject,  that  the  nuclear  substance  is 
present  in  them,  although  it  is  broken  up  into  little  chromatin 
granules  or  chromidia.  They  possess,  therefore,  what  Wilson 
calls  a  scattered  or  distributed  nucleus  ('  The  Cell,'  p.  40), 
and  tKey^  ought  not  to  be  Considered  simply  non-nucleate, 
although  they  seem  to  form  a  kind  of  transition  to  those 
cells  which  contain  a  fully  developed  nucleus.  That  the 
chromidia  in  Protozoa  are  the  biological  equivalents  of  nuclei 
and  only  represent  a  particular  condition  of  nuclear  configura- 
tion has  been  conclusively  proved  lately  by  Fritz  Schaudinn.4 

Oskar_Hertwig,  one  of  the  greatest  biologists  of  the  present 
day,  has  declared  it  to  be  his  opinion  that  really  non-nucleated 
organisms  do  not  exist  ('  Allgemeine  Biologie,  1906,  pp.  44, 
45).  No  actual  facts  can  be  brought  forward  in  support 
of  them,  only  '  various  theoretical  considerations  '  of  a  purely 
speculative  character  ;  as  E.  Hertwig  expresses  it  ('  Lehrbuch 
der  Zoologie,'  7th  ed.  p.  159)  :  '  It  is  easier  to  imagine  that,  in 
spontaneous  generation,  those  organisms  first  came  into  being 
which  consisted  of  only  one  kind  of  substance,  than  those 
in  which  nucleus  and  protoplasm  were  already  distinguished.' 

1  '  Vergleichende  Untersuchung  der  Cytologie  und  der  Bewegungen  der 
Cyanophyceen '   (Contributions  from  the  Botanical    Laboratory,  University  of 
Pennsylvania,  II,  ]904,  pp.  237-306). 

2  '  t)ber  die  Kerne   der  Bakterien '    (Bullet.  International  de  VAcad.  des 
Sciences  de  Bohtme,  VIII,  1903). 

3  '  tiber  den  Kern  der  Bakterien  und  seine  Teilung  '  (Zentralblatt  fur 
Bakteriologie,  XI,  1904,  2nd  Part,  pp.  481-496).     Cf.  the  review  in  the  Natur- 
wissenschaftliche  Rundschau,  XTX,  1904,  No.  29,  pp.  366-369. 

4  '  Neuere  Forschungen  iiber  die  Bef ruchtung  bei  den  Protozoen  '  ( Verhandl. 
der  Deutschen  Zoolog.  Gesellsch.,  1905,  pp.  16-25  and  Plate  I.     See  particularly 


We  cannot,  therefore,  name  any  independent  unicellular 
organism  having  either  a  cell-body  without  a  nucleus,  or  a 
nucleus  without  a  cell-body.  Is  it  possible  that  these  forms, 
so  eagerly  sought  under  Haeckel's  name  cytodes  by  the  up- 
holders of  the  theory  of  spontaneous  generation,  may  occur 
within  the  tissues  of  multicellular  animals  and  plants  ?  If 
they  did  occur,  it  would  prove  nothing  in  support  of  the 
theory  of  spontaneous  generation,  for  once-living  cells  can 
degenerate  and  lose  their  nucleus,  whilst  cells  still  in 
process  of  formation  may  have  a  nucleus  before  the  layer  of 
protoplasm  belonging  to  it  can  be  traced.1  But  in  these  cases 
we  should  have  to  deal  with  the  products  of  living,  nucleated 
cells  ;  not  with  a  spontaneous  coming  into  existence  of  non- 
nucleated  cell-bodies,  or  of  bodiless  nuclei,  out  of  still 
unorganised  primitive  matter.  Let  us  examine  the  facts 
rather  more  closely. 

The  young  red  blood-corpuscles  of  vertebrates  have  a 
nucleus,  which  multiplies  itself  by  direct  division,  and  so 
causes  an  increase  in  the  number  of  red  blood-corpuscles,  as 
we  have  already  stated  (Chapter  V,  pp.  86  and  87).  The 
old  red  blood-corpuscles  lose  their  nuclei  and  become  enucleate, 
but  they  have  ceased  to  be  living  cells,  and  are  only  the  remains 
of  cells  once  alive,  which  still  for  a  time  are  of  use  to  the 
organism  as  bearers  of  the  oxygen  loosely  attached  to  their 
haemoglobin,  but  soon  they  are  dismissed  from  service,  and 
the  white  blood-corpuscles  come  and  devour  them.  The  exist- 
ence of  red  blood-corpuscles  without  nuclei,  accepted  by  most 
authors,2  is  of  no  use  as  evidence  that  there  can  be  living 
cells  without  a  nucleus,  and  that  the  nucleus  is  not,  therefore, 
indispensable  to  the  life  of  the  cell.  Just  as  a  living  cell 
must  have  a  nucleus  or  its  equivalent,  so  a  living  nucleus  must 
have  a  protoplasm  body,  if  it  is  to  continue  in  existence.  It 
is  true  that  there  are  cells  in  which  the  volume  of  the  nucleus 
is  far  greater  than  that  of  the  cell  body.  Spermatozoa  belong 
to  this  class  ;  they  often  have  an  enormous  head  consisting 

1  I  observed  instances  of  this  when  I  was  preparing  the  series  of  sections 
of  Lomechusa  larvae.     They  occurred  during  the  formation  of  new  cenocytes 
in  the  hypodermic  region. 

2  I  say  '  by  most  authors '  for  some  maintain  that  they  have  observed 
nuclei  even  in  old  red  blood-corpuscles.      Cf.  M.  Duval,  Precis   d'Histologie, 
pp.  50,  614,  &c. 


of  the  nucleus  of  the  sperm-cell,  whilst  the  thin  threadlike 
tail  and  probably  also  the  middle-piece,  connecting  it  with  the 
head,  are  the  protoplasmic  elements  of  the  cell ;  but  no  sooner 
has  the  spermatozoon  lost  its  tail  in  the  process  of  fertilisation, 
than  its  existence  as  a  cell  is  over  ;  its  nucleus  perishes,  unless 
it  can  unite  with  a  female  pronucleus  to  form  the  cleavage- 
nucleus  of  the  fertilised  ovum  (cf.  Chapter  VI,  pp.  119,  &c.). 

We  come  now  to  the  reverse  case,  in  which  new  nuclei  are 
formed  apparently  without  a  cell-body.  In  the  history  of  the 
genesis  of  cells,  these  phenomena  play  an  important  part,  as 
we  shall  see  later  on.  This  is  the  so-called  free  nuclear 
formation,  which  is  supposed  to  lead  to  free  cellular  formation. 
These  formations  were  called  free,  because  the  new  nuclei 
were  not  formed  by  division  from  an  old  nucleus,  nor  the 
new  cells  by  division  from  an  old  cell,  but  both  were  supposed 
to  originate  in  an  indifferent  mass  of  protoplasm  called  blastem, 
a  product  of  the  mother-cells  in  the  same  organism.  Such  a 
mode  of  forming  fresh  nuclei,  destined  to  become  the  centres 
of  fresh  cells,  even  if  it  really  existed,  would  have  had 
nothing  to  do  with  spontaneous  generation,  and  it  had  no  real 
existence  at  all.  The  theory  of  free  nuclear  formation  was,  as 
we  shall  see,  to  all  intents  and  purposes  dead  at  the  end  of  the 
nineteenth  century,  and  in  the  twentieth  no  one  can  have 
recourse  to  it  to  support  any  favourite  theory. 

Let  us  now  sum  up  shortly  the  results  of  these  investiga- 
tions. They  amount  to  this :  There  are  no  living  organisms 
simpler  in  organisation  than  the  cell. 

We  can  now  approach  the  question :  '  Is  the  cell  the 
ultimate  unit  of  organic  life,  or  is  it  composed  of  still  lower 
and  more  elementary  units  ?  ' 

According  to  the  laws  of  logic,  we  ought  to  describe  as  the 
lowest  unit  of  life  only  that  part  of  a  morphologically  complex 
living  creature,  which,  at  least  under  certain  conditions,  is 
actually  capable  of  an  independent  existence.  Otherwise 
it  is  no  longer  a  biological  unit,  but  only  a  part  of  a  biological 
unit.  Now  we  have  just  shown  that  no  organism  is  actually 
of  lower  organisation  than  the  cell,  therefore  the  cell  is  actually 
the  lowest  and  ultimate  unit  in  organic  life. 

We  have  seen  moreover,  in  the  previous  sections,  that 
within  the  cell  the  nucleus  and  the  protoplasm  of  the  cell- 


body,  as  well  as  the  morphologically  distinguishable  elements 
of  these  two  chief  parts  of  the  cell,  are  in  no  sense  independent 
of  one  another,  but  are  closely  connected,  so  as  to  make  up 
one  cell,  capable  of  life,  to  which  they  belong  partly  as  essential, 
partly  as  integral  portions.     The  nucleus  is  in  a  certain  degree 
the  material  principle  of  organisation  in  the  cell,  controlling 
its  activities,  but  the  protoplasm  is  indispensable  to  its  life. 
It  is  true  that  the  chromosomes  of  the  nucleus  take  the  leading 
part  in  the  processes  of  cell- division,  fertilisation,  and  trans- 
mission of  qualities,  and  possess  some  amount  of  individuality 
(see  pp.  167,  &c.),  as  they  always  appear  at  the  cell-divisions 
in  definite  shape  and  number,  and  within  these  limits  have  an 
independent  power  to  propagate  themselves  and  develop  by 
means  of  segmentation  and  growth ;  but  still  no  chromosome  can 
exist  and  become  a  nucleus  without  its  corresponding  particle  of 
protoplasm.    And  what  does  this  show  ?    That  the  chromosomes 
are  not  lower  biological  units  within  the   cell,  but  they  are 
merely  essential  morphological  and  physiological  constituents  of 
the  cell.     What  is  true  of  the  chromosomes,  applies  also  to 
the  centrosomes  and  to  all  the  other  less  important  morpho- 
logical elements  of  the  cell.     None  of  them  is  capable  of  inde- 
pendent existence  apart  from  the  cell ;  they  are,  consequently, 
only  parts  of  the  cell,  not  lower  and  more  elementary  units 
out  of  which  the  cell  is  composed  as  a  secondary  formation. 
The   cell,   therefore,   from   the   biological   point   of  view, 
represents  an  indivisible  unit,  although  it  is  composed  morpho- 
logically of  many   different   parts,   whose  various  functions 
co-operate  in  the  one  biological  process  of  life.     The  life  of 
a  multicellular  animal  or  plant  is  one  biological  whole,   in 
which  the  various  organs,  tissues,  and  cells,  with  their  respective 
functions,  all  unite  and  work  together  in  conformity  to  law, 
and  the  discovery  of  the  intercellular  bridges  connecting  the 
various  cells  in  the  body  of  an  animal  or  plant  has  furnished  a 
histological  explanation  of  this  fact,1  and  in  just  the  same 

1  See  Wilson,  The  Cell,  1902,  pp.  59,  60.  An  excellent  account  of  the 
biological  unity  of  the  whole  process  of  growth  and  development  in  the  living 
organism  is  given  by  the  same  author,  pp.  58,  59,  and  393,  &c.  According 
to  him  (p.  59)  cells  are  '  local  centres  of  a  formative  power  pervading  the 
growing  mass  as  a  whole.'  0.  Hertwig  too,  in  his  Allgemeine  Biologic,  1906, 
chapter  xiv,  has  done  much  to  remove  the  obscurity  prevailing  on  the  subject  of 
'  Individuality,'  although  I  am  unable  to  agree  with  him  on  all  points,  e.g. 
in  his  conception  of  personality,'  pp.JJ78  and  383. 


way  the  life  of  a  unicellular  organism  is  an  individual  biological 
unit,  in  spite  of  the  fact  that  the  cell  is  composed  of  various 
parts  with  various  functions.  The  impossibility  of  maintaining 
the  opinion  that  multicellular  organisms  are  mere  aggregations 
of  cells,  has  been  brought  out  very  clearly  by  0.  Whitman 
in  an  article  '  On  the  inadequacy  of  the  cell-theory  of  develop- 
ment '  (Wood's  Hall  Biological  Lectures,  1893). 

The  cell-bridges  forming  protoplasmic  connexions  between 
the  cells  of  the  organism  may,  according  to  Hammar,1  be 
recognised  even  between  the  cleavage-globules  of  the  first 
divisions  of  the  fertilised  ovum.  In  his  '  Allgemeine  Biologie ' 
(1906,  chap,  xiv),  Oskar  Hertwig  stoutly  upholds  the  individual 
unity  of  the  multicellular  organism.  He  distinguishes  clearly 
(p.  371)  two  different  conceptions  of  individuality,  viz.  the 
physiological  and  the  morphological  individual.  The  former 
is  '  an  independent  living  being,'  and  it  is  to  this  alone  that 
the  idea  of  individuality  strictly  speaking  applies.  The 
latter  is  '  a  formal  unit,  which  resembles  a  physiological 
individual  morphologically,  i.e.  in  appearance,  structure,  and 
composition,  but  not  in  the  physiological  sense,  for  it  is  not 
an  independent  living  being,  but  is  taken  as  a  dependent 
part  into  another  higher  physiological  individuality,  or,  in 
other  words,  is  adopted  as  an  anatomical  element  of  the 

The  idea  of  organic  individuality  has  in  recent  times  often 
been  transferred  from  unicellular  organisms  to  every  single 
cell  of  a  multicellular  organism,  so  that  each  cell  in  the  body  of 
an  animal  or  plant  has  been  wrongly  raised  to  the  dignity 
of  an  '  individual,'  although  it  is  not  one  at  all  physiologically, 
i.e.  it  is  not  an  independent  individual,  from  the  biological 
point  of  view,  but  only  a  part  of  an  individual. 

In  just  the  same  way,  in  the  lowest  histological  unit,  viz. 
in  the  morphological  individual  represented  by  the  cell,  the 
part  has  often  been  confused  with  the  whole,  and  attempts 
have  been  made  to  prove,  from  the  composition  of  the  cell, 

1  *  t)ber  eine  allgemein  vorkommende  Protoplasmaverbindung  zwischen 
den  Blastomeren  '  (Archiv  fur  mikroskopische  Anatomie,  XLIX,  1897) ;  '  1st 
die  Verbindung  zwischen  den  Blastomeren  wirklich  protoplasmatisch  und 
primar  ?  '  (ibid.  LV,  1900).  Cf.  also  Korschelt  and  Heider,  Lehrbuch  der 
vergl.  Entwicklungsgesch.,  Jena,  1902,  Allgem.  Teil,  Part  I,  pp.  159,  160. 
On  the  subject  of  intercellular  bridges,  see  also  0.  Hertwig,  Allgemeinf 
Biologie,  pp.  400-406. 


that  there  must  be  organic  units  of  a  lower  order  than  the  cell. 
This  line  of  argument  is  quite  wrong,  and  we  must  clearly  under- 
stand that  we  may  regard  as  the  lowest  units  of  organic  life 
only  those  parts  of  organisms  which,  at  least  under  definite 
conditions — such  as  occur  among  unicellular  animals  and 
plants — are  capable  of  independent  existence.  To  call  the 
parts  of  these  units  '  subordinate  units  '  is  most  deceptive,  for 
they  are  not  units  at  all,  but  only  parts  of  units.  All  the 
arguments  adduced  by  Altmann,  Schlater,  and  other  modern 
writers  against  regarding  the  cell  as  the  final  biological  unit 
are  based  upon  this  quibble.  Flemming  has  shown  this  very 
clearly  with  regard  to  Altmann,  and  says  l  that  evidence  is  still 
inadequate  to  prove  that  Altmann's  granula  are  really  ele- 
mentary organic  units  or  bioblasts,  inasmuch  as  the  chief  point 
in  it  is  absent,  viz.  conclusive  proof  that  one  of  his  famous 
granula  is  capable  of  exercising  its  elementary  vital  functions 
outside  the  cell.  We  arrive  therefore  at  the  same  result  as 
Oskar  Hertwig  in  his '  Allgemeine  Biologie '  (1906,  p.  375),  where 
he  declares  cells  to  be  the  elementary  units  in  the  whole  organic 

If  we  wish  to  find  a  justification  in  fact  for  speaking  of 
'  lower  elementary  units  '  of  living  substance,  we  can  do  so 
only  in  the  sense  in  which  Sachs  spoke  of  energids.  An  energid 
is  a  particle  of  nuclear  substance  with  a  definite  amount  of 
protoplasm  belonging  to  it  and  subject  to  its  control.  In  this 
way  it  would  be  possible  to  avoid  the  difficulties  that  seem  to 
prevent  our  giving  the  same  account  of  cells  with  one  nucleus 
and  of  those  with  more  than  one.  A  cell  with  more  than  one 
nucleus  would  be  made  up  of  a  number  of  energids  not  so 
completely  distinct  from  one  another  as  to  be  called  separate 
cells.  A  cell  with  one  nucleus  would  be  one  fully  developed 
energid.  The  acceptance  of  this  idea  would  obviously  not 
affect  our  opinion  of  the  essential  unity  of  the  cell.  We  may 
even  imagine,  as  Lotsy  does,3  that  the  first  living  beings  were 
monoenergids,  i.e.  very  simply  organised  cells,  consisting  each  of 
a  single  energid.  These  might  swim  about  freely,  but  we  cannot 

1  Cf.  W.  Flemming^  '  tJber  Zellstrukturen '  (Naturwissenschaftliche  Rund- 
schau, XIV,  1899,  No.  35,  p.  444). 

2  To  understand  his  meaning  more  clearly,  see  also  chapter  xvii,  pp.  424, 
&c.,  of  the  same  work. 

3  Biolog.  Zeniralblatt,  1905,  No.  4,  p.  97. 


possibly  imagine  biophors  or  other  '  lower  elementary  units  ' 
to  have  swum  about,  because  they,  as  far  as  they  have  any  real 
existence,  are  only  parts  of  an  energid,  and  not  creatures  capable 
of  independent  life. 

Thus  we  arrive  again  at  the  conclusion  :  The  cell  (or 
energid)  is  actually  the  lowest  unit  in  organic  life.  Therefore 
the  alleged  '  lower  elementary  units  '  of  the  upholders  of  the 
Theory  of  Descent  are  nothing  but  fictions.  It  is  a  matter 
of  complete  indifference  for  this  subject  whether  the  formations 
in  question  can  be  seen  under  the  microscope,  as  definite 
morphological  elements  of  the  cell,  or  whether  they  exist 
solely  as  figments  of  the  imagination  in  the  brain  of  some 
philosophising  naturalist,  for  their  interpretation  as  elementary 
units  is  in  both  cases  equally  imaginary,  although  they  may 
retain  their  significance  as  more  or  less  hypothetical  elementary 
parts  of  the  living  substance. 

I  should  stray  too  far  were  I  to  attempt  to  give  my  readers 
anything  like  a  complete  account  of  the  many  various  theories 
in  which  these  elementary  units  are  concerned.  The  names 
given  to  these  units  by  those  who  believed  they  had  discovered 
them  are  very  numerous.  In  1864  Herbert  Spencer  began 
the  list  by  calling  them  physiological  units  ;  Darwin  called  them 
gemmules,  Erlsberg  and  Ernst  Haeckel  plastidules,  Nageli 
micellae,  Detmer  Lebenseinheiten  or  vital  units,  Hugo  de  Vries 
pangens,  Verworn  biogens,  and  Weismann  biophors,  which  by 
combining  make  up  the  units  next  above  them  or  determinants, 
which  in  their  turn  compose  ids  and  ids  idants.  (Cf .  Chapter  VI, 
pp.  107,  &c.  and  pp.  175,  &c.)  W.  Koux  called  his  elementary 
units  metastructural  parts,  Wiesner  plasomes,  W.  Haacke 
gemmae,  which  he  imagines  as  rhomboid  crystals  lying  side  by 
side  to  form  magnetic  columns  or  gemmaria.1 

L.  Zehnder2  conceives  of  the  elementary  units  of  life  as 
annular  hollow  cylinders,  formed  of  organic  molecules,  and 
he  calls  them  fistellae.  Oskar  Hertwig  calls  his  units  bioblasts,* 

1  For  a  criticism  of  Haacke's  fantastic  '  Doctrine  of  Creation,'  see  my 
article,  '  Zur  neueren  Geschichte  der  Entwicklungslehre  in  Deutschland  :  Eine 
Antwort  auf  W.  Haacke's  Schopfung  des  Menschen,'  Miinster,  1896  (Natur  und 
Offenbarung,  XLII). 

2  Die  Entstehung  des  Lebens  aus  mechanischen   Grundlagen  entwickelt,   I, 
Freiburg  i.  B.,  1899,  pp.  50-52. 

3  Allgemeine  Biologie,  1906,  pp.  52,  &c. 


Simroth :  biocrystals,  and  Altmann  granula,  bioblasts  or  auto 
blasts — granula,  inasmuch  as  they  are  visible  under  the  micro- 
scope as  very  fine  grains  ;  bioblasts,  inasmuch  as  they  re- 
present the  hypothetical  elementary  units  of  the  life  of  the  cell ; 
and  autoblasts,  inasmuch  as  they  are  said  to  be  capable  of  a 
free  existence  outside  the  cell.  It  is  a  pity  that  neither  Altmann 
himself  nor  any  of  his  followers,  among  whom  Gustav  Schlater 
is  conspicuous  for  his  energy,2  have  succeeded  in  demonstrating 
the  existence  of  granula  as  bioblasts  and  autoblasts. 

1  am  far  from  denying  that  the  above-mentioned  theories 
contain  many  ideas  that  are  both  accurate  and  fruitful  for  the 
philosophy  of  life.     (Cf.  0.  Hertwig,   '  Allgemeine   Biologie,' 
chapter  xxxi.) 

Kichard  Hertwig  has  drawn  attention3  to  the  fact  that 
according  to  most  recent  research,  the  chromatin  of  the  cell- 
nucleus  really  possesses  the  properties  which  Nageli  required 
theoretically  for  his  idioplasm  as  the  material  substance  of 
heredity  (1884).  This  hypothetical  substance  in  the  first  place 
must  not  only  be  organised  at  the  time  of  fertilisation,  but  it 
must  have  possessed  its  organisation  beforehand,  and  have 
constantly  preserved  it ;  secondly,  it  must  be  present  in  the 
egg-  and  sperm- cell  in  equal  quantities  ;  and  thirdly,  it  must 
occur  in  all  cells  in  a  state  of  living  metamorphosis,  and  influence 
their  vital  processes.  The  chromosomes  of  the  nucleus  possess 
all  these  properties,  as  I  have  shown  plainly  in  my  account  of 
the  processes  of  cell  division  and  fertilisation  (Chapter  V, 
pp.  123,  &c.  and  pp.  165,  &c.).  That  the  chromatin  of  the  cell- 
nucleus  is  a  real  idioplasm,  a  real  physical  basis  of  inheritance, 
we  must  acknowledge  to  be  extremely  probable  ;  but,  on  the 
other  hand,  it  is  wrong  to  follow  Nageli  in  regarding  the  single 
particles  of  chromatin,  micellae,  as  he  calls  them,  as  elementary 
vital  units  ;  for,  from  their  very  nature,  the  chromosomes  can 
only  be  parts  of  the  nucleus  of  a  living  cell,  with  which  the 
substance  of  inheritance  is  necessarily  connected.  A  living 

*  Bemerkungen  zu  einer  Theorie  des  Lebens  '  ( Verhdndl.  der  Deutschen 
Zoolog.  Gesellsch,  1905,  pp.  214-232). 

2  Cf.    his   articles :     '  Der   gegenwartige   Stand   der   Zellenlehre '    (Biolog. 
Zentralblatt,    XIX,     1899,    Nos.    20-24);      '  Monoblasta— Polyblasta— Poly- 
cellularia  '  (ibid.  XX,  1900,  No.  15). 

3  '  tJber  Befruchtung  und  Konjugation  (Verhandl.  der  Deutschen  Zoolog. 
Qesellsch.,  1892,  p.  101). 


chromosome  apart  from  a  corresponding  particle  of  living 
protoplasm  is  an  impossibility. 

I  will  gladly  acknowledge  that  many  of  these  theories 
of  heredity  display  a  marvellous  wealth  of  ingenuity  and 
intellectual  effort.  This  is  particularly  true  of  Weismann's 
Germ-plasm  theory,  especially  in  the  form  of  the  Theory  of 
Determinants,  in  which  he  stated  it  in  his  lectures  on 
the  Evolution  Theory  in  1892.  It  aims  at  explaining  the 
nature  of  the  germ-plasm,  and  of  all  the  phenomena  of 
heredity,  by  reference  to  particular  structures  and  par- 
ticular distribution  of  even  the  smallest  material  parts  of  the 
germ-plasm.  As  a  general  theory,  however,  it  proves  to  be 

It  seeks  in  a  one-sided  way  to  account  for  the  development 
of  the  individual  out  of  the  preformed  structure  of  most 
minute  material  particles  of  germ,  and  finally  it  is  reduced  to 
the  necessity  of  assuming  the  existence  of  '  vital  affinities  ' 
between  these  minute  particles,  and  this  necessity  reveals  the 
inadequacy  of  the  ingeniously  thought-out  mosaic  theory. 
I  should  prefer  to  accept  Oskar  Hertwig's  Theory  of  Biogenesis 
('  Allgemeine  Biologie,'  chap.xxii,  &c.,  and  especially  pp.  635, 
&c.)  which,  in  a  successful  and  logical  manner,  connects  the  prin- 
ciple of  preformation  with  that  of  epigenesis.  It  too  regards 
the  chromosomes  as  the  material  bearers  of  heredity,  but  takes 
into  account  also  the  dynamic  and  physiological  force  of  their 
interaction  in  the  vital  unity  of  the  whole  process  of  develop- 
ment. If  we  therefore  consider  0.  Hertwig's  hypothetical 
bioblasts  to  be  elementary  particles,  and  not  elementary  units 
of  living  substance,  the  theory  of  biogenesis,  as  a  working 
hypothesis,  is  of  assistance  to  us  in  trying  to  solve  the  problem 
of  evolution.  0.  Hertwig  himself  frequently  emphasises  the 
facts  that  a  cell  containing  a  nucleus  is  the  lowest  morpho- 
logical unit  in  organic  life,  and  that  the  cells  in  multicellular 
organisms  unite  to  form  a  true,  physiological,  living  unit.  On 
p.  569  he  sums  up  his  opinion  as  to  the  causes  of  development 
as  follows  :  '  Continuity  in  development  is  not  attained  by 
means  of  the  emboitement  of  miniature  creatures,  nor  by  the 

1  For  a  criticism  of  it,  see  Y.  Delage,  La  structure  du  protoplasma  et  Us 
theories  fur  Vheredite,  pp.  196,  &c.,  512,  &c.,  667,  &c. ;  also  J.  Keinke,  Philosophic 
der  Botanik,  1905,  pp.  63,  64.  0.  Hertwig,  op.  cit.,  1906,  pp.  361,  452,  &c.,  620, 
633,  &c.  Cf.  also  Chapter  VI,  pp.  174,  &c. 


secretion  of  an  unorganised  formative  material  endowed  with  a 
nisus  formativus,  nor  by  a  substance  composed  of  tiny  germs, 
and  so  to  some  extent  representing  an  extract  of  the  body, 
but  rather  by  the  cell,  a  living  elementary  organism,  which 
by  its  multiplication  and  combinations  gives  rise  to  all  forms 
of  vegetable  and  animal  life.  Continuity  of  organic  develop- 
ment and  of  organic  life  depends  therefore  on  the  principle 
omnis  cellula  ex  cellula.' 

Zoological  and  botanical  research,  whilst  it  has  enlarged  our 
knowledge,  has  tended  more  and  more  to  prove  the  non-exist- 
ence, among  unicellular  organisms,  of  any  that  really  consists 
of  a  simple  lump  of  plasm,  such  as  the  theorists  are  so  anxious 
to  discover.  Fritz  Schaudinn,  who  is  one  of  our  best  authori- 
ties on  Protozoa,  gave  an  address  on  '  Recent  Research  into 
Fertilisation  among  Protozoa '  ('  Neuere  Forschungen  iiber 
die  Befruchtung  bei  Protozoen  ')  at  a  meeting  of  the  German 
Zoological  Society  at  Breslau,  on  June  14,  1905,  and  the 
opinion,  which  he  expressed  in  the  following  resigned  terms, 
must  be  valuable.  He  said  :  '  As  in  the  class  of  Flagellata, 
universally  regarded  as  one  of  the  lowest  groups  of  Protozoa,  the 
study  of  the  problem  of  fertilisation  alone  shows  the  finer 
structures  of  the  cell  to  be  almost  as  highly  differentiated  and 
complicated  as  in  the  highest  organisms,  the  discovery  among 
Protozoa  of  our  day  of  that  tiny  drop  of  simple  plasm,  whence 
the  animal  cell  is  supposed  to  have  originated,  may  present 
some  difficulties.' 


The  question  as  to  the  lowest  actual  units  of  organic  life 
is  closely  connected  with  the  question  whether  spontaneous 
generation  is  possible. 

The  Monists  assure  us  that  it  is  undoubtedly  possible, 
because  it  must  have  taken  place  ;  organic  life  exists  now  in  the 
world,  and  yet  there  was  a  time  when  it  did  not  exist,  as  the 
world  was  still  in  a  state  of  molten  heat.  Therefore  there  must 
have  been  an  epoch  when,  under  particularly  favourable 
chemico-physical  conditions,  the  first  primordial  plasm  or 
plasms  were  produced  from  inorganic  combinations  of  carbon. 
The  assumption  of  spontaneous  generation  is  therefore  an 


indispensable  postulate  of  science,  according  to  Monism.  M. 
Verworn,  the  eminent  physiologist,1  argues  in  the  following 
way  in  favour  of  spontaneous  generation  :  '  Living  substance  is 
actually  a  part  of  the  matter  composing  our  world.  The  com- 
bination of  this  matter  to  form  a  living  substance  was  as  much 
a  necessary  result  of  the  evolution  of  the  world  as  the  formation 
of  water,  viz.  a  necessary  result  of  the  gradual  cooling  of 
those  masses  which  made  up  the  crust  of  the  earth.  In 
the  same  way  the  chemical,  physical,  and  morphological 
properties  of  living  substance,  as  we  know  it,  are  the 
inevitable  consequence  of  the  working  of  our  present  ex- 
terior conditions  of  life  upon  the  interior  conditions  of  earlier 
living  substance.  Interior  and  exterior  conditions  of  life 
stand  in  inseparable  interaction,  and  the  expression  of  it 
is  life.'  Thus  the  assumption  of  spontaneous  generation  is 
scientifically  irrefutable  ! 

What  are  we  to  say  in  answer  to  this  demand  made  upon 
us  in  the  name  of  science  ?  I  am  quite  ready  to  admit  that  the 
first  organisms  were  made  of  inorganic  matter,  for,  if  they 
were  not,  they  would  have  to  be  created  out  of  nothing,  which 
I  am  by  no  means  inclined  to  believe.  But  the  theory  _of 
spontaiieousgeneration  requires  inorganic  matter  to.,  have 
first  organisms  by  itself  and  ont.  pf  its  own 
resources.  Thelatter  assumption  cannot  be  a  '  postulate 
oT  science,'  because,  as  I  shall  show,  it  plainly  contradicts 
actual  facts.  If  I  were  to  maintain,  on  the  contrary,  that  the 
first  living  beings  were  brought  forth  from  matter  still  not 
organised,2  under  the  action  of  a  higher  power  proceeding  from 
the  Creator  of  matter,  I  should  have  given  up  the  idea  of 
spontaneous  generation,  and  have  replaced  it  by  that  of 
creation  in  the  wider  sense.  I  say  '  creation  in  the  wider 
sense,'  because  the  matter  out  of  which  the  organisms  were 
formed  already  existed,  and  the  creative  action  was  limited 
to  the  organisation  of  this  matter.  It  is  quite  indifferent  to 
our  question  how  we  imagine  this  organisation  to  have  taken 
place,  whether  it  was  by  an  eductio  formarum  e  potentia 

1  Allgemeine  Physiologic,  1901,  pp.  333,  &c. 

2  The  antithesis  is  between  organised   and  not  organised,   not  between 
organic  and  inorganic,  for  many  organic  substances,  i.e.  such  as  under  natural 
conditions  are  formed  only  in  living  organisms,  can  be  made  artificially  in 
chemical  laboratories. 


materiae,  or  by  some  other  method  ;  nor  do  we  know  when  the 
first  organisation  of  matter  occurred.1 

It  is  obvious  that  the  material  basis  for  the  origin  of  the 
first  forms  of  life  must  be  supplied  by  definite  arrangements  of 
atoms  and  the  physical  and  chemical  laws  governing  them  ; 
but  this  no  more  proves  spontaneous  generation  to  have  taken 
place  than  does  the  fact  that  also  at  the  present  time  the 
phenomena  of  life  rest  on  a  chemico-physical  foundation. 

The  problem  with  which  we  are  now  concerned  is  therefore 
the  following  :  '  What  are  we  to  think  of  the  theory  of  spon- 
taneous generation,  which  requires  lifeless  matter  of  itself 
to  have  produced  the  •  first  living  organisms  ?  '  We  must 
examine  the  scientific  character  of  this  spontaneous  generation 
more  closely.2  We  may  disregard  those  rash  and  untenable 
theories  which,  like  Ernst  Haeckel's  carbon  theory,  aim  at 
giving  a  direct  account  of  spontaneous  generation.  It  is  im- 
possible not  to  be  amazed  at  the  audacity  with  which  these 
hypotheses  are  published  as  being  the  results  of  scientific 
research.  For  instance,  in  1892,  S^haafflaussn  seriously 
asserted  that  water,  air,  and  various  mineral  substances  had 
united  directly  under  the  influence  of  light  and  heat,  and  had 
produced  a  colourless  Protococcus,  which  afterwards  turned 
into  the  Protococcus  viridis.  Yves  Delage  remarks  somewhat 
sarcastically : 8  'If  the  matter  is  so  simple,  why  does  not  the 
author  produce  a  few  specimens  of  this  protococcus  in  his 
laboratory  ?  We  'would  gladly  supply  him  with  the  necessary 
chlorophyll.'  Still  more  fantastic  is  Haeckel's  discovery 

1  Hamann  (Darwinismus  und  EntwicJclungslehre,  1892,  p.  58)  and  Fechner 
assume  that  matter  was  originally  in  a  '  cosmo-organic  '  state,  subject  to  the 
laws  of  neither  organic  nor  inorganic  nature,  but  this  hardly  seems  to  be  a 
tenable  hypothesis,  for  the  chemico-physical  laws  governing  the  atoms  and 
molecules  in  matter  can  scarcely  have  differed  from  those  that  now  govern 
inorganic  matter,  and,  in  the  same  way,  the  mechanical  laws  governing  the 
movement  of  atoms,  molecules,  and  masses  must  have  been  identical  with  the 
present  laws.     It  follows  that  primitive  matter  in  itself  must  be  judged  accord- 
ing to  the  laws  of  the  present  inorganic  world,  and  so  the  ability  to  produce 
organisms  spontaneously  cannot  have  belonged  to  its  essence. 

2  On    the   differences    between     living    creatures     and    lifeless    matter 
see  also  L.  Dressel,  Der  belebte  und  der  uribelebte  Rtoff,  Freiburg  i.  B.,   1883. 
I    cannot    here   discuss  the   other  reasons   for  declaring  the   theory  to  be 
philosophically   untenable.     Stolzle   remarks   very   justly    (A.    v.    Kolliker's 
6  Stellung  zur  Deszendenzlehre,'  1901,  p.  14)  that  as  an  explanation  the  theory 
of  spontaneous  generation  is  worthless,  if  for  no  other  reason,   because  it 
attempts  to  explain  the  unknown,  not  by  the  known,  but  by  another  unknown. 

3  La  structure  du  protoplasma  et  les  theories  sur  rherediie,  p.  402. 

o  2 


of  an  organic  primitive  pulp  to  which  he  gave  the  classical 
name  of  Autoplasson,  or  self-forming  substance.  We  have 
already  seen  how  badly  Bathybius  Haeckelii  has  fared,  which 
was  supposed  to  be  the  first  real  representative  of  this  pulp. 
On  a  level  with  Haeckel's  autoplasson  is  the  plastic  primary 
substance  discovered  in  1874  by  an  Italian  named  Maggi,  who 
called  it  Gliq,  and  declared  it  to  be  the  starting-point  of  the 
development  of  the  organic  world.  It  does  not  altogether 
savour  of  genuine  science. 

Thoughtful  naturalists  cannot  regard  as  serious  such 
clumsy  attempts  to  solve  the  most  delicate  problems  ;  it  is 
obvious  that  they  are  doomed  to  be  failures.  The  chemical 
composition  of  nucleinic  acid,1  which  is  present  chiefly 
in  the  chromatin  (nuclein)  of  the  nucleus,  and  is  therefore 
intimately  connected  with  the  problem  of  heredity,  defies 
all  the  attempts  made  hitherto,  and  likely  to  be  made  in  future, 
by  the  upholders  of  the  carbon  theory  to  explain  its  chemical 
formula  C26H49N9P3022.  That  it  is  a  hopeless  task  to  seek 
the  origin  of  life  directly  from  inorganic  matter  is  acknow- 
ledged frankly  by  most  naturalists.  If  theories,  such  as 
Haeckel's  carbon  theory,  are  still  brought  forward,  it  is  not  for 
the  benefit  of  really  scientific  circles,  but  that  the  so-called 
'  general '  readers  may  be  disposed  thereby  to  accept  a  realistic 
and  monistic  view  of  life. 

I  have,  of  course,  no  intention  of  condemning  the  ingenious 
attempts,  which  chemists  are  making  with  ever-increasing 
success,  to  produce  organic  matter  artificially  in  their  labora- 
tories. By  means  of  unwearied  industry,  Emil  Fischer  and 
other  eminent  workers  in  this  department  of  research  have 
advanced  steadily  towards  mastering  the  chemical  construc- 
tion of  a  molecule  of  albumen,3  and,  perhaps,  erelong  the 

1  For  a  detailed  account  of  the  chemistry  of  the  nucleus  see  Dr.  Hans 
Malfatti,  Zur   Chemie  des  Zellkerns  :    reprinted   from  the  Berichte  des  natur- 
wissenschaftlich-medizin.    Vereins    in    Innsbruck    (XXII,    1891-2).       Cf.    also 
Hof meister,  '  "Dber  den  Bau  des  Eiweissmolekiils  '  ( Verhandl.  der  74  Versamm- 
lung  Deutscher  Naturforscher  zu  Karlsbad,  1902,  communicated  to  the  Natur- 
wissenschaftliche  Rundschau,  1902,  No.  42).     Also  Wilson,   The   Cell,  pp.  41, 
330,  &c. ;   0.  Hertwig,  Allgemeine  Biologie,  1906,  pp.  29,  &o. 

2  Cf.  Karl  Kautzsch,  '  Uber  das   Eiweiss,  insbesondere  die  neuesten  For- 
schungen  auf  dem  Gebiete  der  Eiweisschemie '  (Natur  und  Schule,  V,  1905, 
pp.  195-208) ;    G.  v.  Bunge,  Lehrbuch  der  Physiologic  des  Menschen,  II,  1905, 
pp.  55-70  ;    Fr.  Samuely,  '  Die  neueren  Forschungen  auf  dem  Gebiete  der 
Eiweisschemie  und  ihre  Bedeutung  fur  die  Physiologic'  (Biolog.  Zentralblatt, 
1906,  Nos.  11,  12,  13-15)  ;   0.  Hertwig,  Allgemeine  Biologie,  chapters  ii,  iii. 


artificial  synthesis  of  the  simplest  forms  of  albumen  will  be 
accomplished  by  these  indefatigable  students.  But  this  would 
prove  nothing  about  spontaneous  generation.  The  albumen 
molecules,  with  their  highly  complicated  chemical  composition, 
are  the  constituents  of  living  creatures,  but  even  in  the  smallest 
cell  these  constituents  are  alive,  and  no  astute  human  in- 
telligence will  ever  succeed  in  breathing  the  breath  of  life, 
capacity  for  growth  and  propagation,  into  one  of  these  arti- 
ficially prepared,  lifeless  molecules  of  albumen,  and  still  less 
can  chance  ever  have  been  in  a  position  to  form  molecules  of 
albumen  by  itself.  Oskar  Hertwig  remarks  very  aptly  in  his 
'  Allgemeine  Biologie '  (1906,  p.  19)  :  *  Even  if  chemistry  in 
course  of  time  were  able  to  produce  artificially  by  synthesis 
all  existing  forms  of  albumen — to  undertake  to  form  a  proto- 
plasmic body  would  still  resemble  Wagner's  attempt  to 
crystallise  out_a_homunculus  in  ajtest-tube.' 

Modern  physics  will  in  vain  strive  to  do  what  organic 
chemistry  fails  to  accomplish.  It  is  not  long  since  people 
believed  that  the  discovery  of  radium  had  removed  the  hindrance 
which  had  frustrated  all  previous  attempts  to  produce  life.1 

On  June  30,  1905,  John  Butler  Burke,  of  the  Cavendish 
Laboratory  in  Cambridge,  startled  the  scientific  world  by 
announcing  that,  with  the  help  of  radium,  he  had  succeeded 
in  producing  from  sterilised  bouillon  a  substance  that  showed 
certain  signs  of  life  :  the  first  living  albumen  body  appeared 
to  have  been  born  artificially  !  But  it  was  unhappily  a  mis- 
carriage. Sir  William  Eamsay,  the  famous  physicist  and 
investigator  of  the  properties  of  radium,  soon  explained  what 
Burke  had  observed,  and  accounted  for  it  in  a  very  simple 
way.  The  powdered  radium,  which  Burke  had  strewn  upon 
the  bouillon,  produced  in  it  chemical  changes.  The  emanation 
of  the  radium  decomposes  the  water  in  the  bouillon  into 
oxygen  and  hydrogen,  and  has  at  the  same  time  the  peculiarity 
of  coagulating  albumen.  Consequently  this  emanation  could 
not  fail  to  form,  in  any  watery  fluid  containing  albumen, 
little  bubbles  of  gas  surrounded  by  a  covering  of  coagulated 
albumen.  As  more  gas  is  produced,  these  bubbles  increase 
and  occupy  more  space,  so  as  to  present  the  appearance  of  a 
very  small,  growing  organism.  In  reality,  therefore,  this 

1  '  Das  Radium  und  die  Urzeugung  '  (Oaea,  XLII,  1906,  Part  I,  pp.  34-36). 


alleged  living  creature  was  nothing  but  a  lifeless  covering  of 
albumen  filled  with  gas  !  This  explains  a  phenomenon  observed 
by  Burke,  viz.  that  the  new-born  organism  melted  away  in 
the  water,  for  the  water  gradually  removed  the  gelatine  from 
the  '  cell- walls,'  and  they  returned  to  lifeless  non-existence. 

We  cannot  waste  time  here  on  the  refutation  of  the  various 
old  and  new  theories  of  spontaneous  generation ;  we  will 
rather  turn  our  attention  to  the  attempts  made  by  scientific 
men  to  present  the  problem  of  spontaneous  generation  in  a 
1  more  comprehensible  or  more  acceptable  '  form.  To  this 
category  belong  the  theories  that  have  devised  the  simplest 
possible  elementary  units  of  life,  in  order  by  their  means  to 
bridge  over  the  chasm  between  the  atoms  and  molecules  of  the 
inorganic  world  and  the  simplest  forms  of  life  ;  or,  if  the  chasm 
cannot  be  actually  bridged,  they  aim  at  diminishing  its  width 
to  such  an  extent  that  a  bold  '  stroke  of  genius  '  may  help 
them  over  it.  To  leap  from  inorganic  matter,  or  even  from 
artificially  produced  organic  combinations,  to  the  living  cell 
is  a  very  hazardous  proceeding,  which  even  the  most  daring 
advocate  of  the  theory  of  evolution  would  hesitate  to  venture 
upon.  Therefore  there  is  only  one  way  of  getting  over  the 
difficulty.  The  chasm  must  be  crossed,  not  at  One  bound, 
but  by  degrees,  and  so  intermediate  halting-places  are  neces- 
sary. These  hypothetical  intermediate  stations  are  called 
'  simpler  elementary  units  of  life  ' ;  they  are  used  to  make  up 
the  phylogeny  of  the  cell  by  means  of  the  assertion  that  nature 
has  taken  these  steps  before  us,  in  order  to  produce  the  first 
cell  out  of  inorganic  matter.  In  this  way  the  theory  of  spon- 
taneous generation  is  supposed  to  be  made  more  acceptable 
from  the  scientific  point  of  view. 

The  statement  just  given  is  not  an  invention  of  my  own, 
it  is  only  a  short  summary  of  the  way  in  which  Gustav  Schlater 
in  the  Biologisches  Zentralblatt  for  1899  (pp.  729,  &c.)  tries  to 
give  a  phylogenetic  value  to  Altmann's  granular  theory. 
Schlater  thinks  that  Altmann's  newly  discovered  elementary 
units  are  of  great  importance,  chiefly  because  they  bring  us 
nearer  to  a  comprehension  of  spontaneous  generation.  He 
says  on  this  subject  (p.  732)  :  '  Although  at  the  present  time 
we  are  naturally  not  yet  in  a  position  to  fix  the  moment  when, 
through  a  complicated  molecule  of  albumen,  the  first  ray  of 


life  flashed,  which  changed  the  dead  molecule  into  a  living 
organism,  or,  let  us  say,  into  an  autoblast ;  nevertheless  such 
a  change  is  much  more  within  our  comprehension  than  such 
a  gigantic  transition  in  evolution,  as  that  from  a  dead  molecule 
of  albumen  to  a  complicated  organism  like  the  cell.' 

There  must  have  been  a  flash  somewhere  for  life  to  have 
begun  at  all ;  even  Schlater  acknowledges  this.  But  it  is 
eventually  a  matter  of  perfect  indifference  whether  the  flash 
was  at  the  spontaneous  generation  of  an  autoblast  or  of  a  cell  ; 
the  flash  of  the  first  spark  of  life  in  lifeless  matter  is  as  in- 
explicable in  the  one  case  as  in  the  other.  Schlater  might  have 
saved  himself  the  trouble  of  writing  over  a  hundred  pages  in 
support  of  bioblasts  and  autoblasts,  for  by  so  doing  he  has 
quite  gratuitously  brought  himself  into  conflict  with  scientific 
facts,  which  know  nothing  of  autoblasts,  i.e.  of  Altmann's 
granules  with  a  free  and  independent  existence,  but  recognise 
cells  as  the  lowest  units  of  organic  life.  He  has  brought  himself 
needlessly  into  conflict  with  scientific  laws  of  thought,  which 
forbid  us  to  regard  Altmann's  granules  as  bioblasts,  i.e.  as  real 
elementary  units  of  life,  because  they  are  actually  only  biologi- 
cally dependent  parts  of  the  real  biological  units,  viz.  the  cells. 
So  Schlater's  whole  argument  misses  its  point.  He  has  not 
succeeded  in  establishing  the  existence  of  elementary  units, 
having  a  lower  degree  of  organisation  than  the  cell ;  nor  has 
he  succeeded  in  explaining  the  origin  of  life,  even  by  assuming 
the  existence  of  these  units.  The  summa  summarum  is  in 
his  case  another  unmistakable  breakdown  of  the  theory  of 
spontaneous  generation. 

Therefore  in  1899  the  theory  did  not  fare  better  than  in 
the  previous  contests  that  it  had  had  to  undergo.  It  has  always 
suffered  defeat,  and  as  scientific  research  advances,  it  with- 
draws into  obscurity.  It  may,  perhaps,  be  interesting  to  give 
my  readers  a  short  sketch  in  broad  outlines  of  this  retreat  of 
the  theory  of  spontaneous  generation. 

There  was  a  time  when  generatio  aequivoca  or  spontanea 
was  regarded  not  only  as  possible,  but  as  of  actual  occurrence. 
This  was  during  the  so-called  '  dark  ages '  and  the  still 
darker  mediaeval  period.  At  that  time  men  believed  that 
the  origin  of  organic  beings  was  influenced  to  a  great  extent 
by  the  stars.  I  am  not  referring  to  the  dreams  of  astrologers, 


but  to  the  Aristotelian  theory  of  the  formation  of  new  organic 
beings  from  decaying  substances,  the  cause  of  which  was 
supposed  to  be  a  mysterious  power  proceeding  from  the 
heavenly  bodies.  This  ancient  theory  of  spontaneous  genera- 
tion is  far  less  contrary  to  common  sense  than  the  modern 
theory,  and  considering  the  state  of  scientific  knowledge  at 
the  time  was  far  more  pardonable.  It  was  taken  up  in  very 
various  ways  by  the  naturalists,  poets,  and  quacks  of  the  period. 
As  an  example  I  may  refer  to  Vergil's  '  Georgics,'  where  there 
is  a^recipejor  making  bees.  A  dead  ox  is  to  be  laid  out,  beaten 
vigorously,  and  left  to  decompose  in  its  hide,  until  the  bees 
develop  in  its  body.  Vergil  did  not  draw  upon  his  imagination 
when  he  gave  this  recipe  ;  it  is  based  upon  real  observations 
wrongly  interpreted.  There  are  some  robber  flies  that 
resemble  bees  very  closely,  belonging  to  the  genus  Eristalis, 
the  larvae  of  which  develop  in  decomposing  matter.  It  might 
easily  escape  the  notice  of  a  casual  observer  that  the  old  flies 
had  already  laid  their  eggs  there.  Even  the  famous  ant-stone, 
lapis  myrmecias,  which  was  supposed  to  grow  in  ants'  nests 
and  to  combine  the  nature  of  the  ant  with  that  of  a  precious 
jewel,  able  to  cure  various  ailments  among  mankind,  is  no 
mere  fiction.  The  story  originated  in  the  discovery  in  ants' 
nests  of  the  cocoons  of  the  rose  chafer  (Cetonia  floricola)  which, 
when  the  beetle  has  developed,  really  contain  a  living  jewel 
of  a  golden  or  emerald  green  colour,  in  a  covering  of  the  size 
of  a  pigeon's  egg,  formed  of  earth.1 

As  methods  of  observation  improved  in  modern  times, 
the  theory  of  spontaneous  generation  gradually  lost  favour. 
As  early  as  the  seventeenth  and  eighteenth  centuries  it  was 
challenged  by  naturalists,  such  as  Eedi,  Malpighi,  Swammerdam 
and  Reaumur,  and  was  pushed  into  the  background,  although 
in  the  nineteenth  century  it  had  some  champions  who  defended 
it  theoretically.  In  the  middle  of  the  nineteenth  century 
much  was  done  to  overthrow  it  by  von  Siebold  and  E.  Leuckart 
in  the  department  of  parasites,  by  Ehrenberg  in  that  of  In- 
fusoria, and  by  de  Bary,  and  especially  by  Pasteur  in  that  of 
Bacteria.  Thus  modern  scientific  research  has  removed  one 
support  after  another  from  the  theory  of  spontaneous  genera- 

1  Of.  Lochner  v.  Hummelstein,   '  Lapis  myrmecias  falsus,  cantharidibus 
gravidus  '  (Ephem.  Ac.  Nat.  Curios,  1687,  Observ.  ccxv,  436-441). 


tion,  until  now  nothing  is  left  of  it — except  that  it  is  'a  postulate 
of  science.' 

As  early  as  1651  an  Englishman/17  William  Harvey, 
formulated  the  famous  principle  Omne  vivum  ex  ovo,  in  his 
work  '  De  generatione  animalium.'  In  this  form  the  dictum 
is  not  universally  true,  for  the  unicellular  organisms  multiply 
themselves  not  hy  eggs,  but  by  cell-division  or  gemmation, 
which  is,  however,  only  a  special  form  of  cell- division  (see 
p.  86).  Therefore  Harvey's  saying  must  be  amended  and 
receive  the  form :  Omne  vivum  ex  vivo.  It  was  not  until  two 
hundred  years  later  that  Rudolf  Virchow,  the  founder  of  cellular 
pathology,  in  1858  set  the  modern  axiom  of  biology,  Omnis 
cellula  ex  cellula,  beside  Harvey's  dictum. 

The  theory  of  spontaneous  generation  found  for  a  time 
its  last  refuge  in  just  that  cellular  theory  which  subsequently 
dealt  it  its  death-blow.  In  order  to  account  for  the  origin 
of  the  cell,  Schwann  propounded  his  Cj/toUa^tema  theory, 
according  to  which  cell-formation  took  place  by  way  of  a  sort 
of  crystallising  process  in  matter  still  unorganised.  The  first 
deposit  from  the  primitive  matter  or  cytoblastema  was, 
according  to  Schwann,  the  nucleolus  of  the  cell,  round  which 
a  membrane  formed  ;  between  the  nucleolus  and  the  membrane 
a  fluid  penetrated  by  endosmosis,  so  forming  the  cell-nucleus ; 
round  this  again  there  was  a  second  membrane,  and  by  endos- 
mosis more  fluid  made  its  way  between  this  membrane  and 
the  nucleus,  so  that  finally  the  membrane  enclosed  the  cell, 
having  in  its  centre  the  nucleus  with  the  nucleolus. 
Schwann  imagined  the  cell  to  have  been  formed  in  this  way 
spontaneously  out  of  unorganised  matter  by  generatio 
aequivoca.  It  was  a  most  ingenious  idea,  but  it  did  not 
correspond  with  facts,  and  it  soon  had  to  be  given  up. 

The  somewhat  later  blastem  theory  advanced  by  Charles 
Eobin,  a  French  scientist,  has  this  advantage  over  Schwann's 
cytoblastema  theory,  that  it  does  not  assume  the  formation  of 
cells  out  of  unorganised  matter.  Kobin's  blastems,  which 
give  rise  to  new  cells,  are  the  product  of  previously  existing 
cells  of  the  same  organism.  It  is,  therefore,  not  correct  here 
to  speak  of  a  generatio  aequivoca.  Kobin's  theory  was  nearer, 
to  the  process  that  really  goes  on  in  cell-formation  in  another 
respect  also,  for  he  thought  that  the  nucleus  of  the  new  cell 


was  formed  before  the  nucleolus.  Bound  the  nucleus  a  layer 
of  protoplasm  took  up  its  position  and  finally  surrounded 
itself  with  a  membrane.  This  account  of  the  genesis  of  the 
cell  also  failed  to  agree  with  ascertained  facts.  It  is  true  that 
for  a  considerable  time  it  found  much  support  in  the  embryonic 
development  of  insects.  Hugo  von  Mohl  had  proved  that 
free  cell-formation  did  not  occur  among  plants,  and  Albert  von 
Kolliker  had  proved  its  non-occurrence  among  animals  ;  it 
had  long  been  established  that  among  higher  animals  the  blasto- 
derm of  the  embryo  had  its  origin  in  continued  cell-division 
from  the  cleavage-nucleus  produced  by  the  union  of  the  egg- 
and  sperm-nuclei,  and  yet  for  some  time  it  seemed  that  among 
insects  there  was  free  cell-formation  in  Robin's  sense.  In 
1864,  in  his  classical  studies  on  the  development  of  Diptera, 
August  Weismann  still  felt  bound  to  uphold  this  theory  of 
free  cell-formation,  as  he  could  not  perceive  any  processes  of 
cell- division  in  the  formation  of  the  blastoderm  in  these 
insects.  As  recently  as  1888  Henking  l  thought  that  he  had 
found  that  the  nuclei  of  the  blastoderm  in  the  egg  of  Musca 
were  not  formed  by  division  from  the  cleavage-nucleus,  but 
by  free  nuclear  formation  in  the  isolated  particles  of  plasm 
dispersed  among  the  masses  of  yolk. 

On  this  subject  Korschelt  and  Heider  remark  in  their 
excellent '  Lehrbuch  der  vergleichenden  Entwicklungsgeschichte 
der  wirbellosen  Tiere'  (special  section,  part  2,  Jena,  1892,  p. 
764),  that  this  opinion  seems  to  be  quite  untenable.  In  those 
insect  eggs  which  are  so  extraordinarily  rich  in  nutritive  yolk 
(deuteroplasm)  as  are  the  eggs  of  flies,  the  processes  of  cell- 
division  are  very  apt  to  escape  observation  under  the  micro- 
scope. In  other  insect  eggs  that  contain  less  yolk  (such  as 
those  of  the  plant-louse,  gall-gnat  and  gall-fly),  these  processes 
have  undoubtedly  been  observed,  and  we  must  take  the 
latter,  rather  than  the  former,  as  illustrating  the  normal 
course  of  blastoderm  formation  in  the  eggs  of  insects.  Thus 
the  last  support  of  free  cellular  formation  has  been  removed, 
and  we  now  have  a  general  law  that,  not  only  does  every  new 
cell  arise  out  of  a  previously  existing  cell,  but  each  new  nucleus 
out  of  a  previously  existing  nucleus. 

*  Die  ersten  Entwicklungsvorgange  im  Fliegenei  und  freie  Kernbildung  ' 
(Zeitschrift  fur  wissenschajtliche  Zoologie,  XLVI). 


Walter  Flemming  in  1882  added  the  third  dictum,  Omnis 
nucleus  ex  nucleo,  to  the  two  biological  axioms  laid  down  by 
Harvey  and  Virchow  respectively.  As  Boveri's  theory  of 
the  individuality  of  the  chromosomes  (see  p.  167)  is  constantly 
receiving  fresh  confirmation,  we  must  add  yet  a  fourth  dictum, 
dating  from  1903,  viz. :  Omne  chromosoma  e  cJiromosomate. 
In  it  the  antagonism  shown  by  modern  biology  to  the  theory 
of  spontaneous  generation  has  reached  its  climax.  The  four 
axioms — Omne  vivum  ex  vivo,  Omnis  cellula  ex  cellula, 
Omnis  nucleus  ex  nucleo,  Omne  chromosoma  e  chromosomate — 
have  destroyed  the  theory  as  far  as  modern  naturalists  are 
concerned.  It  can  continue  to  exist  only  outside  the  sphere 
of  scientific  thought. 

Very  descriptive  of  the  scientific  weakness  of  the  theory 
of  spontaneous  generation  are  the  following  remarks  which 
occur  in  the  famous  biologist,  Oskar  Hertwig's  '  Allgemeine 
Biologie  '  (1906,  p.  263)  :  '  Considering  the  state  of  natural 
science  at  this  time,  there  seems  but  little  prospect  that  any  one 
engaged  in  scientific  research  will  succeed  in  artificially  pro- 
ducing even  the  simplest  living  organism  from  lifeless  material. 
He  has  certainly  no  more  hope  of  success  than  Wagner  in  Goethe's 
"  Faust  "  had  in  his  attempts  to  brew  a  homunculus  in  a  retort.' 

J.  Eeinke,  the  distinguished  botanist,  has  expressed 
himself  much  more  sharply  still  on  the  subject  of  the  theory 
of  spontaneous  generation,  in  many  places  in  his  works.1 

It  is,  therefore,  an  absolutely  necessary  consequence  that 
organic  life  on  earth  did  not  begin  by  way  of  spontaneous 
generation,  and  that  it  is  altogether  unscientific  to  represent 
this  theory  as  a  postulate  of  science,  in  spite  of  its  being  quite 
untenable.  Our  modern  evolutionists  above  all  others  lay 
great  stress  upon  the  fact  that  the  laws  of  nature  now  existing 

1  See  his  book  Die  Welt  ah  Tat  (Berlin,  1899),  the  third  edition  of  which 
appeared  in  1903.  In  it  J.  Keinke  devotes  a  chapter,  almost  thirty  pages 
in  length,  to  proving  the  impossibility  of  spontaneous  generation,  and  he 
deduces  from  this  argument  the  conclusion  that  we  shall_neyer  be  able  IP 
account  for  the  origin  of  organic  life  unless  we  accept  the  creation^  In  1905 
a  fourth  edition  was  published]  Ci  also  J.  .kemke's  Einleitung  in  die  theo- 
retische  Biologie,  1901,  pp.  555-562,  and  his  treatise  '  Der  Ursprung  des  Lebens 
auf  der  Erde '  (Turmer  Jahrbuch  1903) ;  also  his  inaugural  oration  at  the 
International  Botanical  Congress  in  Vienna,  June  12,  1905,  '  Hypothesen, 
Voraus^etzungen,  Probleme  in  der  Biologie  '  (Biolog.  Zentralblatt,  XXV,  1905, 
No.  13,  pp.  433-446),  pp.  442,  443.  He  has  an  excellent  refutation  of  the 
hypothesis  of  spontaneous  generation  in  his  last  book,  Philosophie  der  Botanik 
(Leipzig,  1905),  chapter  xii,  On  the  Origin  of  Life. 


must  have  existed  from  the  beginning,  and  that  we  must 
regard  them  as  a  safe  standard,  applicable  also  to  the  most 
remote  history  of  the  animal  and  vegetable  world,  if  we  wish 
to  solve  the  problem  of  descent  scientifically.  It  is  quite  in 
vain  that  they  appeal  to  the  *  uniform  causal  connexion  of 
natural  phenomena  '  to  support  the  theory  of  spontaneous 
generation.  J.  Reinke  says  very  aptly  ('  Einleitung  in  die 
theoretische  Biologie,'  p.  558)  :  '  I  am  of  opinion  .  .  .  that 
the  assumption  of  spontaneous  generation  in  past  ages  agreed 
no  more  with  our  ideas  of  causality  than  a  hypothesis  that  a 
million  years  ago  water  flowed  uphill  of  its  own  accord  would 
agree  with  them.'  And  in  another  place  he  says  ('  Philosophic 
der  Botanik,'  p.  188)  :  '  Just  as  at  no  stage  of  the  earth's 
cooling  was  it  possible  for  two  lines  to  form  a  triangle,  so  was 
it  never  possible  for  an  organism  of  the  most  primitive  kind 
to  be  produced  by  the  forces  and  combinations  of  inorganic 
matter.'  There  is  therefore,  as  Reinke  rightly  points  out, 
scarcely  a  greater  incongruity  possible,  than  for  one  and  the 
same  man  to  reject  spontaneous  generation,  as  a  thoughtful 
naturalist,  and  in  the  same  breath  to  declare  it  to  be  a  postulate 
of  science,  when  he  speaks  as  a  philosophical  thinker.  What 
is  a  '  postulate  of  science  ?  '  This  name  can  properly  be 
given  only  to  a  truth  that  proceeds  logically  from  facts,  and 
never  to  a  hypothesis  that  is  in  antagonism  to  them. 

From  this  point  of  view,  what  true  postulate  of  science  is 
there  to  account  for  the  first  origin  of  organic  life  ? 

Life  cannot  always  have  existed  on  our  earth  ;  modern 
cosmogony  leaves  us  no  room  to  doubt  this,  for  it  teaches 
us  that  the  earth  was  once  in  a  condition  of  molten  heat. 
How,  then,  did  the  first  organisms  come  into  being  ? 

It  is  an  unprofitable  amusement  to  fancy,  with  Thomson 
and  Helmholtz,  that  they  were  brought  by  meteors  from  other 
planets,  or,  with  H.  E.  Richter  and  Arrhenius,  that  they  fell 
upon  the  earth  as  cosmic  dust,1  for  life  must  have  had  a  beginning 

1  In  his  Einleitung  in  die  theoretische  Biologie,  p.  559,  Reinke  says  :  '  Men 
like  Lord  Kelvin  (Thomson)  and  Helmholtz  would  not  have  devised  their 
hypothesis  of  the  advent  of  primitive  cells  from  other  planets,  if  they  had 
not  regarded  spontaneous  generation  as  lost  beyond  all  hope  of  recovery.' 
It  should  be  noticed  that  Thomson  has  repeatedly  and  decidedly  said  that 
we  must  assume  the  existence  of  a  Creator.  Cf.  Karl  Kneller,  S.J.,  Das 
Christentum  und  die  Vertreter  der  neueren  Naturwissenschaft,  Freiburg  i.  B., 
1903,  pp.  28-30,  and  The  American  Quarterly  Review,  XXVIII,  1903,  p.  603. 


on  the  planets  of  other  solar  sj^stems  also,  since  they   too 
are  subject  to  the  same  cosmogonic  laws. 

Therefore,  how  did  the  first  organisms  come  into  being  ? 
Every  effect  must  have  an  adequate  cause.  Inorganic  matter 
cannot  have  been  this  cause,  for  science  teaches  us  this  when 
she  declares  spontaneous  generation  to  be  contradictory  to 
facts.  But  at  that  time  there  was  still  nothing  in  the  world 
but  inorganic  matter  and  its  laws.  Therefore  there  must 
have  been  some  cause  extraneous  to  this  world,  which  brought 
forth  the  first  organisms  out  of  matter.  This  cause,  extraneous 
to  the  world,  and  differing  substantially  from  it,  in  spite  of  its 
omnipresence  in  it,  is  an  intelligent  cause,  and  is  the  personal 
Creator,  so  often  denied  and  feared  by  modern  monism. 

Monism,  in  its  desire  to  get  rid  more  easily  of  the  theistic 
conception  of  God,  has  caricatured  it,  until  finally  the  Creator 
has  been  represented  as  a  '  gaseous  vertebrate '  (Haeckel), 
bearing  alarming  testimony  to  its  discoverer's  want  of  philo- 
sophical knowledge.  The  new  idea  of  God  invented  by  monism, 
and  set  up  in  place  of  the  personal  Creator,  is  nothing  but  a 
fantastic  sort  of  idol  draped  in  a  covering  of  theism  to  hide  its 
atheistic  nakedness.  Everything  acceptable  in  the  monistic 
idea  is  borrowed  from  theism,  the  omnipresence  of  God  in 
nature,  His  action  in  all  creation,  &c.  But  what  is  peculiar 
to  monism,  and  marks  it  off  from  theism,  is  the  theory  of  the 
substantial  identity  of  God  and  the  world,  which  is  nonsense 
from  the  philosophical  point  of  view.  A  god  identical  with 
the  world,  and  developing  himself  through TtTisTnot  an  infinitely 
perfect  being,  having  the  reason  of_hia_£xistence  always  in 
himself,  but  he  is  a  mass  of  imperfections  and  contradictions . 
Any  thoughtful  student  of  nature  must  be  able  to  see  this 
for  himself. 

We  may  therefore  close  our  examination  of  the  theory 
of  spontaneous  generation  with  the  following  statement : 
Organic  life  has  not  always  existed  in  our  world,  nor  can  it 
have  originated  by  itself  from  inorganic  matter.  Natural 
science  brings  us  thus  far  ;  and  natural  philosophy  leads  us 
on  to  the  further  irrefutable  conclusion : — It  follows  that 
some  cause  superior  to  the  world  produced  the  first  organisms 
from  lifeless  matter.  When  and  how  this  took  place  is  perfectly 
indifferent,  as  far  as  the  necessity  of  this  conclusion  is  concerned. 


Even  if  we  did  not  need  to  assume  the  existence  of  any  special 
vital  principle,  and  if  the  living  atoms  differed  from  inorganic 
matter  only  by  being  in  a  state  of  movement  peculiar  to  them- 
selves, we  could  still  not  dispense  with  the  Creator  to  create 
primitive  matter,  and  to  impart  to  those  atoms  their  state  of 
movement,  in  order  thereby  to  make  them  the  constituents  of 
the  first  living  creatures.  But  we  are  still  more  forcibly 
constrained  to  acknowledge  the  existence  of  a  personal  Creator 
by  the  fact  that  modern  science  proves,  more  and  more  clearly, 
that  all  vital  processes  are  subject  to  their  own  particular  law, 
and  we  are  thus  compelled  to  accept  the  entelechies,  or  formal 
principles,  which  raise  the  laws  governing  inorganic  matter 
to  a  higher,  vital  conformity  to  law  in  the  case  of  living 

Thus  the  acceptance  of  a  personal  Creator  is  seen  to  be  a 
real  '  postulate  of  science.'  For,  as  J.  Keinke  rightly  points 
out  :  '  If  we  assume  at  all  that  living  creatures  once  were 
formed  of  inorganic  matter,  as  far  as  I  can  see,  the  theory  of 
creation  is  the  only  one  which  satisfies  the  demands  of  logic  and 
causality,  and  so  satisfies  those  of  reasonable  scientific  research.'1 

1  Einleitung  in  die  theoretische  Biologie,  p.  559.  See  also  the  quotations  from 
Charles  Darwin  and  Lyell  on  the  indispensability  of  a  creation  in  Chapter  IX, 
at  the  end  of  §  6. 





Mechanics  and  physiology  of  development  (p.  211).  History  of  the 
theories  of  preformation  and  epigenesis  (p.  214).  Germ-regions  for 
the  formation  of  organs  and  isotropy  of  the  egg-plasm  (p.  216). 


Self  -  differentiation  and  dependent  differentiation  (p.  219).  0. 
Hertwig's  directly  formative  influence  of  exterior  causes,  and 
Zur  Strassen's  criticism  of  this  theory  (p.  220).  The  respective 
functions  of  preformation  and  epigenesis  in  ontogeny.  Driesch's 
prospective  potency  and  equipotential  systems  (p.  225). 




Epigenetic  evolution  (p.  235).  Differential  or  integral  division  of 
the  nuclear  substance  ?  (p.  236).  The  machine  theory  or  vitalism  ? 
(p.  238).  Inadequacy  of  the  machine  theory  of  life  (p.  238). 
Driesch's  experiments  on  Clavellina  (p.  245).  The  problem  of  life 
demands  a  vitalistic  solution  (p.  247). 


LIFE  is  for  the  student  of  nature  a  fact  which  he  must  take  as 
his  starting  point  for  the  further  investigation  of  the  pheno- 
mena of  life.  All  attempts  to  account  for  the  origin  of  life 
from  inorganic  matter  by  way  of  spontaneous  generation  have 
failed,  as  they  contradict  what  modern  cytology  teaches. 
This  has  been  shown  clearly  in  Chapter  VII.  Organic  chemistry 
may  make  a  bold  and  triumphant  advance  by  means  of  the 
laborious  and  ingenious  experiments,  by  which  she  examines 
the  elementary  composition  of  living  organisms  and  the 
chemical  processes  of  their  metabolism.  She  may  succeed 
eventually  in  producing  synthetically  a  highly  complex  mole- 
cule of  albumen  in  her  test-tubes,  but  one  thing  will  always 
be  wanting  to  the  artificial  product,  viz.  life. 

The  laws  of  inorganic  matter  apply  also  to  living  creatures, 
but  in  their  case  the  laws  are  subordinate  to  a  higher  unity, 
which  brings  their  activities  into  that  wonderful  harmony, 
tending  to  fulfil  a  purpose,  that  we  call  a  vital  process. 



Even  in  the  simplest  unicellular  organisms,  Amoebae  and 
Bacteria,  we  encounter  the  mysterious  problem  of  life.  We 
meet  with  it  in  a  more  astonishing  form  in  the  fertilised  egg- 
cell,  out  of  which  a  multicellular  plant  or  animal  is  produced 
by  a  long  series  of  cell- divisions.  In  Chapter  VI  we  have 
traced  the  microscopical  processes  that  go  on  within  the 
germ-cells,  before  their  union  in  the  fertilised  ovum.  Now  let 
us  consider  the  following  deeply  important  questions  con- 
cerning the  continuation  of  the  same  great  problem  of  life  : — 

1.  How  does  the  organism,  in  its  individual  ontogeny,  de- 

velop from  the  egg-cell  into  a  perfect  animal  or  plant  ? 

2.  How  have  the  organisms  on  our  earth  been  evolved, 

each  according  to  its  kind,  from  the  first  appearance 
of  life  in  the  world  to  our  present  Fauna  and  Flora? 

In  this  chapter  we  can  deal  only  with  the  first  of  these 
questions,  leaving  the  other  for  subsequent  discussion. 

It  will  conduce  to  a  better  understanding  of  the  following 
arguments  if  we  begin  by  studying  the  chief  kinds  of  cleavage 
in  animal  ova.1 

After  fertilisation  is  effected,  the  egg-cell  divides  rapidly 
into  2, 4,  8,  16,  &c.,  cells,  which  become  smaller  as  the  process 
of  cleavage  continues.  These  cells  are  called  cleavage-spheres 
or  Uastomeres.  We  speak  of  each  egg  as  having  an  animal 
and  a  vegetative  pole,  inasmuch  as  the  substance  at  one  pole 
serves  chiefly  to  form  the  animal  organs  or  nervous  system, 
and  that  at  the  other  pole  serves  chiefly  to  form  the  vegetative 
organs  or  digestive  tract. 

Different  types  of  cleavage  processes  are  distinguished  ; 
the  peculiarities  of  which  depend  upon  the  quantity  of  food — • 
yolk  or  deuteroplasm  in  the  egg,  and  upon  its  position. 

The  cleavage  of  the  egg  is  total  or  partial,  according  as  the 
whole  substance  of  the  egg,  or  only  part  of  it,  undergoes  the 
process  of  cleavage.  It  is  total  in  eggs  poor  in  yolk,  partial  in 
those  rich  in  yolk,  as  the  yolk  impedes  cleavage.  In  total 
cleavage  the  whole  substance  of  the  egg  is  used  to  build  up 
the  embryo,  and  therefore  eggs  that  show  this  type  of  cleavage 
are  called  holoblastic,  whilst  those  with  partial  cleavage  are 
called  meroblastic.  In  holoblastic  eggs  with  total  cleavage, 

1  For  further  details  see  R.  Her  twig,  Lehrbuch  der  Zoologie,  1905,  pp  125,  &c. 
(Eng.  trans,  pp.  151,  &c.), 


it  is  either  equal  or  unequal,  according  as  the  cleavage-spheres 
are  equal  or  unequal  in  size ;  this  depends  upon  the  quantity 
of  yolk  in  the  egg. 

In  meroblastic  eggs  with  partial  cleavage,  it  is  either 
discoidal  or  superficial.  This  distinction  depends  upon  the 
position  of  the  yolk  in  the  egg.  If  the  yolk  is  accumulated 
about  the  vegetative  pole,  the  cleavage  is  limited  to  the  animal 
polo  (discoidal  cleavage)  ;  if  the  yolk  lies  in  the  centre  of  the 
egg,  only  the  surface  of  the  egg  shows  a  thin  layer  of  cleavage 
cells  surrounding  the  unsegmented  central  mass  (superficial 

The  eggs  which  have  their  yolk  more  or  less  concentrated 
at  the  vegetative  pole  are  called  telolecithal ;  those  with  a 
mass  of  yolk  in  the  centre  are  called  centrolecithal. 

Superficial  cleavage  occurs  among  arthropoda  and  especially 
among  insects.  Discoidal  cleavage  occurs  in  birds  and  in 
most  of  the  other  vertebrates,  among  molluscs,  also  in  cuttle- 
fish, and  in  some  Arthropoda  and  Tunicata.  Equal  and 
unequal  cleavage,  however,  may  appear  in  all  kinds  of  multi- 
cellular  animals. 

The  account  just  given  of  the  different  types  of  cleavage 
does  not  depend  immediately  upon  the  question  whether 
preformation  or  epigenesis  controls  the  cleavage  of  the  egg. 
We  shall  have  to  study  the  behaviour  of  animal  eggs  towards 
these  two  factors  in  development  in  the  third  part  of  this 


It  was  chiefly  through  Karl  Ernst  von  Baer  (1791-1876) 
that  the  study  of  the  individual  development  of  animals  became 
a  special  branch  of  zoology,  to  which  the  name  ontogeny  has 
been  given.  There  is  an  analogous  branch  of  botany,  dealing 
with  the  individual  development  of  plants.1 

Both  confront  us  with  the  same  old  and  yet  ever  new 
questions  with  which  from  remote  antiquity  the  minds  of 
ordinary  men  have  busied  themselves,  no  less  than  the  inquiring 
spirit  of  the  scholar.  Why  are  children  like  their  parents  ? 

1  See  the  general  sketch  of  the  departments  of  biological  science,  Chapter  I, 
pp.  3,  &c. 



Why  does  an  oak  always  grow  out  of  an  acorn,  and  why  is  a 
chicken  always  hatched  out  of  a  hen's  egg  ?  Whence  comes 
the  specific  conformity  to  law  in  accordance  with  which,  from 
the  fertilised  egg  of  any  given  species,  there  is  invariably  pro- 
duced a  being  similar  to  that  which  gave  life  to  the  egg-cell  ? 
What  is  the  influence  determining  the  germ  of  the  new  indi- 
vidual to  follow  one  line  of  development  rather  than  another  ? 
Moreover,  are  the  laws  controlling  this  development  purely 
mechanical,  or  are  there  also  vital  laws,  essentially  superior 
to  what  goes  on  in  inanimate  nature  ? 

These  are  undoubtedly  very  interesting  and  important 
questions,  having  a  bearing  not  only  upon  biological  research, 
which  is  seeking  to  solve  the  problem  of  life  by  way  of  natural 
science,  but  also  upon  philosophy,  which  is  striving  to  penetrate 
into  the  essential  nature  of  life  by  means  of  the  phenomena 
of  life. 

We  stand  therefore  face  to  face  with  the  problem  of  deter- 
mination, i.e.  with  the  question  :  '  What  are  the  causes  con- 
trQlling_embr¥onic  development  ?  '  Regarded  from  afar  this 
problem  may  scorn  to  the  layman  to  resemble  a  porcupine 
bristling  with  all  manner  of  technical  difficulties,  so  that  an 
ordinary  intellect  can  scarcely  venture  to  approach  it.  Let  me, 
however,  see  if  I  cannot  succeed  in  inducing  this  porcupine 
of  the  problem  of  determination  to  lay  down  his  prickles,  and 
show  himself  to  my  readers  in  a  harmless  form,  presenting  no 
particular  difficulties  to  a  man  of  average  intelligence. 

To  begin  with,  I  must  follow  Oskar  Hertwig  l  in  pointing 
out  that  a  one-sided  view  of  the  subject  cannot  fail  to  be  a 
false  one.  Many  internal  and  external  causes  co-operate  in 
the  development  of  organic  beings,  and  they  do  so  in  such  a 
way  that  the  internal  causes  are  invariably  the  foundation  for 
the  action  of  the  external  factors. 

The  problem  that  we  have  to  discuss  is  closely  connected 
with  the  subject  of  Chapter  VI,  viz.  the  relation  of  the  pro- 
cesses of  cell-division  to  the  problems  of  fertilisation  and 
heredity.  We  came  to  the  conclusion  that  the  chromatin 
constituents  of  the  nuclei  of  the  germ-cells,  that  is  to  say  their 
chromosomes,  might  with  great  probability  be  regarded  as  the 
chief  material  bearers  of  the  phenomena  of  heredity,  and 

1  Allgemeine  Biologie,  pp.  132,  &c.,  and  138,  &c. 


consequently  also  as  the  chief  bearers  of  the  laws  governing 
the  particular  development  of  each  kind  of  animal  and  plant. 

Yet  in  making  this  statement,  we  have  alluded  to  only 
one  side  of  the  problem  of  organic  development,  viz.  to  that 
which  is  the  subject  of  microscopical  cytology.  We  now 
encounter  a  series  of  other  questions  which  are  of  great  interest 
as  affecting  the  problem  of  life  : — Does  the  development  of^tho 
fertilised  ovum  depend  upon  a  self-differentiation,  controlled 
exclusively  by  the  interior  factors  already  present  in  the  egg, 
or  does  it  depend  upon  a  differentiation  controlled  chiefly  _by 
exterior  causes  ?  Must  we  uphold  the  theory  of  preformation, 
which  assumes  that  there  is  in  the  egg  a  foreshadowing  of  the 
whole  future  being,  or  the  theory  of  epigenesis,  which  asserts 
that  the  organs  of  the  embryo  are  formed  afresh  in  the  course 
of  its  development  ?  The  so-called  problem  of  determination 
is  comprised  in  the  answers  to  these  questions.  It  will  be  well 
to  show  shortly  what  success  has  hitherto  attended  the  attempts 
made  to  solve  it.  Incidentally  we  shall  have  to  be  careful  to 
ascertain  whether  the  individual  development  of  organic  beings 
is  controlled  by  some  special  laws  of  life,  as  vitalism  asserts,  or 
whether  it  can  be  satisfactorily  explained,  as  the  mechanics 
theory  maintains,  by  merely  chemico -physical  causes. 

The  branch  of  biology  that  deals  with  experimental  research 
into  the  laws  and  causes  of  organic  formation  is  known  as  the 
physiology  of  development.  Wilhelm  Koux,  the  principal 
founder  of  this  branch  of  science,  called  it  '  mechanics  of 
development.'  But  as  the  mechanical  explanation  of  the  pro- 
cesses under  consideration  is  only  a  part  of  the  problem,  we 
agree  with  Hans  Driesch,  who  has  done  excellent  work  in  this 
department  of  research,  that  it  is  better  to  adopt  the  name 
physiology  of  development.1 

1  Among  the  publications  bearing  on  this  subject  I  may  mention  particularly 
Das  Archiv  fur  Entwicklungsmechanik  der  Organismen,  edited  by  W.  Roux 
in  Halle  a.  S.  Also  W.  Roux,  '  Einleitung  zu  den  Beitragen  zur  Entwick- 
lungsmechanik des  Embryo '  (Zeitschrift  fur  Biologic,  XXI,  1885) ;  Die 
tinttvicklungsmechanik  der  Organismen,  eine  anatomische  Wissenscha/t  der 
Zukunft,  Vienna,  1890 ;  Die  Entwicklungsmechanik,  ein  neuer  Zweig  der 
biologischen  Wissenschaft,  Leipzig,  1905.  E.  Pfliiger,  '  t)ber  den  Einfluss  der 
Sehwerkraft  auf  die  Teilung  der  Zellen  und  auf  die  Entwicklung  des  Embryo  ' 
(Archiv  fiir  die  gesamte  Physiologie,  XXXII,  1883) ;  '  Beitrage  zur  Entwick- 
lungsmechanik des  Embryo ' :  No.  1.  '  Zur  Orientierung  iiber  einige  Probleme 
der  embryonaleii  Entwicklung  '  (Zeitschrift  fur  Biologie,  XXI,  1885) ;  '  tJber 
die  Bestimmung  der  Hauptrichtungen  des  Froschembryo  im  Ei  und  iiber  die 

p  2 


It  may  appear  to  some  readers  that  these  questions  have 
already  been  answered  satisfactorily  by  the  results  previously 
described  of  microscopic  morphology.  Among  the  higher 
organisms  at  least,  under  normal  circumstances,  the  development 
of  a  new  individual  can  result  only  from  fertilisation,  which 
consists  essentially  in  the  union  of  the  nuclei  of  the  ovum  and 
spermatozoon,  as  we  saw  at  the  end  of  Chapter  VI  (pp.  156,  &c.). 
As  the  chromosomes  of  the  nuclei  of  the  germ-cells  are  the 
bearers  of  heredity,  visible  under  the  microscope  and  passing  in 
definite  number  and  order  from  the  parents  to  the  children, 
and  as  (according  to  Boveri's  theory  of  the  individuality  of  the 
chromosomes)  they  preserve  some  amount  of  independence 
during  the  whole  process  of  development,  it  may  seem  a 
superfluous  question  to  ask  whether  the  development  of  the 
fertilised  ovum  depends  upon  preformation  or  epigenesis, 
upon  an  independent  or  a  dependent  differentiation.  Has  not 

erste  Teilung  des  Froscheis  '  (Breslauer  drztliche  Zeitschrift,  1885).  0.  Hertwig, 
'  tiber  den  Wert  der  ersten  Furchungszellen  fiir  die  Organbildung  des  Embryo  ' 
(Archiv  fiir  mikroskopische  Anatomie,  XLII,  1893) ;  Zeit-  und  Streilfragen  der 
Biologie,  I,  Jena,  1894 ;  Prdformation  oder  Epigenese  ?  II,  1897  ;  Mechanik 
und  Biologie  ;  Die  Zelle  und  die  Gewebe,  II,  Jena,  1898  ;  Allgemeine  Biologie, 
Jena,  1906  (especially  recommended).  A.  Weismann,  Das  Keimplasma, 
Jena,  1892 ;  Vortrdge  uber  Deszendenztheorie,  Jena,  1902  (Lectures  on  the  Theory 
of  Evolution,  Eng.  trans.).  E.  B.  Wilson,  *  Amphioxus  and  the  Mosaic  Theory 
of  Development '  (Journal  of  Morphology,  VIII,  1893).  H.  E.  Crampton, 

*  Experimental  Studies  of  Gastropod  Development '  (Archiv  fiir  Entwicklungs- 
mechanik,  III,  1896).     C.  0.  Whitman,  'Evolution  and  Epigenesis'  (Wood's 
Hall  Biological  Lectures,  1894).     Hans  Driesch,  Analytische  Theorie  der  organ- 
iftchen  Entwicklung,  Leipzig,  1894 ;  Die  orga nischen  Regulationen,  Leipzig,  1901 ; 

*  Kritisches  und  Polemisches  '  (Biolog.  Zentralblatt,  XXII,  1902,  Nos.  5,  6,  14, 
15  ;    XXIII,  1903,  Nos.  21-23) ;    '  Ergebnisse  der  neueren  Lebensforschung  ' 
(Politisch-Anthropologische  Revue,  II,  1903,  part  10).      0.  Herbst,  Formative 
Reize  in  der  tierischen  Ontogenesis,  Leipzig,  1901.     Th.  H.  Morgan,  Regeneration, 
New  York  and  London,   1901.     0.  L.  Zur  Strassen,  '  tiber  das  Wesen  der 
tierischen   Formbildung '    (VerhandL  der   Deutschen   Zoolog.    Gesellsch.,  1898, 
pp.  142-156).  K,  Heider, '  Das  Determinationsproblem '  ( VerhandL  der  Deutschen 
Zoolog.  Gesellsch.,  1900,  pp.  45-97).     L.  Kathariner,  *  Uber  die  bedingte  Unab- 
hangigkeit  der  Entwicklung  des  polar  differenzierten  Eis  von  der  Schwerkraf t ' 
(Archiv  fiir   Entwicklung 'smechanik,  XII,  1901,  part  4) ;    '  Weitere  Versuche 
iiber  die  Selbstdifferenzierung  des  Froscheis  '  (Ibid.  XIV,  1902,  parts  1  and  2) ; 
'  Schwerkraf twirkung  oder  Selbstdifferenzierung  ? '  (Ibid.  XVIII,   1904,  part 
3,  pp.  404-414).     An  excellent  general  account  of  the  problem  of  Determination 
is  given  by  Korschelt  and  Heider  in  their  Lehrbuch  der  Entwicklungsgeschichte 
der  wirbeltosen  Tiere,  Allgem.  Teil,  I,  Jena,  1902,  §  1,  cf.  especially  chapter  ii, 
4  Das  Determinationsproblem  '  (pp.  81-150).     In  the  same  book  will  be  found 
a  list  of  all  the  literature  on  the  subject  up  to  the  year  1902 ;  for  works  published 
since  that  date  see  0.  Hertwig,  Allgemeine   Biologie.      Of   botanical   works 
dealing  with  embryology  I  may  mention  particularly  :   W.  Pfeffer,  Pflanzen- 
physiologic,  I,  Leipzig,   1897  ;    II,  first  part,   1901.     Also  G.   Klebs,   « Uber 
Probleme  der  Entwicklung '  (Biolog.  Zentralblatt,  XXXIV,  1904,  Nos.  8,  9,  14, 
15,  16,  &c.). 


this  question  been  already  answered  in  what  has  gone  before,  and 
have  we  not  already  decided  in  favour  of  preformation,  and 
of  independent  differentiation  ? 

The  matter  is  not  so  simple  as  it  appears.  Even  if  we 
assume  that  the  chromosomes  of  the  nuclei  of  the  germ-cells 
are  the  chief  material  bearers  of  heredity,  passing  on  from  one 
generation  to  another,  we  still  have  to  solve  the  problem  of 
the  development  of  the  organism  from  the  fertilised  ovum. 
This  difficult  question  still  remains :  *  What  causes  the  groups 
of  cells,  formed  out  of  one  egg-cell  by  cleavage-division,  to 
differ  from,  one  another  more  and  more,  both  morphologically 
and  physiologically,  as  the  development  of  the  embryo  pro- 
ceeds ?  How  is  it  that  these  groups  of  cells  develop  into  the 
various  tissues  and  organs  of  one  and  the  same  individual  ? ' 
In  other  words :  *  What  causes  underlie  the  process  of  harmo- 
nious differentiation,  by  means  of  which  the  wonderful  and 
complicated  structure  of  the  complete  organism  with  all 
its  manifold  parts  is  produced  from  the  apparently  simple 
ovum  ? ' 

The  physiology  of  development,  which  we  now  have  to 
study,  approaches  this  problem  on  lines  quite  unlike  those 
followed  by  microscopical  anatomy.  The  latter  has  recourse 
to  modern  methods  of  staining  and  cutting  sections,  and 
examines  the  tissues  and  cells  of  animals  under  the  strongest 
microscopes,  and  strives  to  trace  all  the  morphological  changes 
in  the  nucleus  and  cytoplasm  of  the  cells,  but  the  former 
proceeds  by  way  of  actual  experiment.  It  takes,  for  instance, 
the  living  ovum  of  a  frog,  subjects  it  to  all  possible  kinds  of 
artificial  treatment,  to  pressure,  twisting,  division  or  partial 
destruction  of  its  cleavage-spheres,  and  then  observes  how 
the  embryo  develops  under  these  conditions.  From 
these  observations  it  draws  its  conclusions  regarding  the 
laws  and  causes  of  the  embryonic  development  of  living 

It  proceeds  also  to  study  the  course  of  regeneration  in  the 
living  organism  by  similar  methods.  It  tries  experimentally, 
in  the  case  of  a  creature  that  has  reached  an  advanced  stage 
of  development,  how  far,  and  in  what  way,  the  faculty  is  re- 
tained of  forming  afresh  lost  organs  and  tissues.  The  experi- 
ments made  by  G.  Wolff  and  others  with  a  view  to  determining 


the  power  of  regeneration  in  the  lens  of  the  eye  of  a  salamander 
have  become  particularly  famous.1 

Before  we  discuss  the  results  of  modern  research  in  em- 
bryology, we  must  refer  shortly  to  the  previous  history  of  the 
problem  of  determination.3 

The  question  whether  the  future  individual  is  contained 
in  the  egg,  and,  if  so,  under  what  form,  has  aroused  the  interest 
of  students  in  all  ages,  although  until  recent  times  there  has 
been  very  little  certain  knowledge  upon  which  to  found  any 
theory.  In  the  seventeenth  and  eighteenth  centuries  the 
most  eminent  scientists,  such  as  Swammerdam,  Malpighi, 
Leeuwenhoek,  Haller,  Bonnet  and  Spallanzani  declared  them- 
selves to  be  in  favour  of  the  preformation  theory,  then  known 
as  the  doctrine  of  evolution,  or  unfolding.3 

They  observed  the  development  of  the  butterfly  in  the  pupa, 
and  the  blossom  in  the  bud,  and  laid  down  the  dictum :  '  Evolu- 
tion is  merely  the  unfolding  of  parts  already  present  in  the  egg- 
or  sperm-cell,  but  imperceptible  to  us  by  reason  of  their 
diminutive  size  and  transparency.'  It  is  true  that  we  can 
trace  in  the  pupa  all  the  organs  of  the  future  butterfly,  and  in 
the  ripe  bud  all  the  parts  of  the  future  flower,  but  when  this 
theory  of  unfolding  is  applied  to  the  embryonic  development 
of  living  creatures,  it  leads  to  very  peculiar  results.  According 
to  it,  in  the  first  ovum  of  each  species  4  all  the  individuals  of 
all  the  succeeding  generations  must  have  been  contained  in 
infinite  numbers  and  in  infinitely  diminutive  size.  For  instance, 
the  ova  of  the  first  cat  must  have  contained  extremely  small 
editions  of  all  the  future  cats  that  would  ever  be  born  to  the 

1  G.  Wolfi,Entwicklungsphi/siologische8ludien,I,  1895;  Die  Regeneration  der 
UrodelenUnse.     Cf.  also  Part  II,  1901,  and  Part  III,  1905,  of  the  same  series 
of   studies  in  the  Arcliiv  fur  EnttvicMungsmechanik.     Hans  Spemann,    '  Uber 
Linsenbildung  nach  experimenteller  Entfernung  der  primaren  Linsenbildungs- 
zellen  '  (Zoolog.  Anzeiger,  XXVIII,  1905,  No.  11,  pp.  419-432).     A  list  of  the 
other  works  on  this  subject  by  Barfurth,   Colucci,   Fischel,  Herbst,   Lewis, 
Mencl,  E.  Miiller,  Schaper  and  Spemann  will  be  found  in  Spemann,  p.  432. 
Cf.  also  0.  Hertwig,  Allgemeine  Biologie,  pp.  546,  &c. 

2  Cf.  0.  Hertwig,  Allgemeine  Biologie,  pp.  350,  &c. 

3  At  the  present  day  we  generally  speak  of  the  theory  of  evolution  with 
reference  to  the  evolution  of  the  species,  not  with  reference  to  that  of  the 
individual.     In  order  to  avoid  confusion,  I  have  used  the  expression  '  theory 
of  preformation  '  to  designate  the  theory  of  evolution  in  the  earlier  sense. 

4  Or  in  the  first  spermatozoon,  for,  according  to  the  theory  of  the  animal- 
culists,  it  was  not  the  egg-cell,  but  the  sperm-cell,  which  transmitted  hereditary 
qualities.     See  p.  104  and  p.  158. 


end  of  the  world.  This  has  also  been  termed  the  theory  of 

In  1759  Kaspar  Friedrich  Wolff  in  his  '  Theoria  generationis ' 
for  the  first  time  opposed  the  old  theory  of  preformation,  and 
by  so  doing  became  the  founder  of  the  theory  of  epigenesis. 
After  a  careful  examination  of  the  development  of  a  chicken, 
he  came  to  the  conclusion  that  the  egg  was  only  a  mass  of 
unorganised  matter,  which  was  gradually  organised  in  the 
course  of  the  development  of  the  embryo.  Wolff's  opinion 
is  right  to  this  extent,  that  the  organs  of  the  embryo  are  really 
formed  anew,  because  the  fertilised  egg  (as  was  recognised 
only  in  the  nineteenth  century)  still  has  the  character  of  a 
simple  cell,  and  so  cannot  consist  of  organs.  But  Wolff  was 
wrong  in  thinking  the  egg  a  mere  mass  of  unorganised  matter, 
for  modern  microscopical  research  has  revealed  to  us  the 
wonderfully  delicate  structure  of  the  egg-cell  and  its  nucleus, 
and  has  shown  us  the  chromosomes,  which,  being  definite 
parts  of  the  nucleus,  are  the  material  bearers  of  heredity,  and 
are  distributed  with  such  marvellous  exactitude  among  the 
cleavage-cells  of  the  egg  as  it  develops.  I  will  not,  however, 
at  this  point  anticipate  the  historical  development  of  the 
problem  of  determination. 

As  the  study  of  embryology  advanced  in  the  first  half  of  the 
nineteenth  century,  the  theory  of  epigenesis  found  increasing 
favour,  and  soon  became  predominant. 

In  1853,  Eudolf  Leuckart,  a  famous  zoologist,  wrote 
in  his  article  on  procreation  :  *  Our  knowledge  of  the  develop- 
ment of  the  embryo  and  of  the  formation  of  the '  procreative 
substance  admits  of  only  one  interpretation,  viz.  in  the  sense 
of  epigenesis — there  can  be  no  further  doubt  on  the  subject ; 
the  embryo  is  the  product  of  a  new  formation  in  connexion 
with  the  procreative  substance.' 

As  late  as  the  year  1872,  Ernst  Haeckel  in  his  *  Anthropo- 
geny  '  described  the  human  embryo  in  the  so-called  monerula 
stage l  as  a  '  completely  homogeneous,  structureless  mass,' 

1  We  owe  the  '  discovery  '  of  this  stage  in  the  embryonic  development 
of  man  to  a  mistake  on  Haeckel's  part.  He  believed,  though  wrongly,  that 
the  germinal  vesicle  of  the  embryo  broke  up  as  soon  as  embryonic  development 
began.  According  to  Haeckel's  fanciful  anthropogeny,  the  monerula  stage 
in  the  human  germ  is  a  lineal  repetition  of  the  monera  stage  of  our  most  remote 
ancestors.  As  a  matter  of  fact,  not  only  is  this  monera  stage  existent  only 


as  a '  simple  lump  of  primitive  matter.'  Haeckel  must  certainly 
have  studied  the  human  embryo  through  very  cloudy  glasses,  if 
in  the  year  1872  he  was  still  able  to  see  so  little  of  its  finer 
histological  structure,  although  Goette  fared  no  better  in  1875, 
when  he  studied  the  egg  of  the  toad,  and  declared  it  to  be  an 
unorganised  lifeless  mass,  produced  by  a  transformation  of 
one  or  more  germ- cells. 

The  theory  of  epigenesis,  however,  was  not  destined  to 
stand  its  ground  much  longer.  As  microscopes  became  more 
perfect,  both  the  ovum  and  the  spermatozoon  were  seen  to 
contain  elements  of  very  complicated  composition,  which  had 
to  prepare,  by  a  special  process  of  maturation,  for  the 
union  of  their  nuclear  substances,  effected  by  fertilisation. 
At  once  the  breath  of  popular  favour  veered  round  to  the 
preformation  theory,  although  it  was  no  longer  the  old  theory 
of  emboitement,  but  assumed  an  entirely  new  form. 

In  1874  Wilhelm  His l  propounded  the  theory  of  there  being 
gerrnregions  or  local  areas  for  the  formation  of  organs  in 
theTincRviduaT.  development  of  vertebrates.2  According  to  this 
theory  definite  tracts  in  the  fertilised  ovum  are,  in  virtue 
of  some  special  interior  tendency  or  Arilage,  destined  to 
form  definite  organs  in  the  embryo.  At  the  same  time  he 
submitted  Haeckel's  fantastic  ideas  on  human  embryology 
to  a  most  destructive  criticism  in  his  article.  The  new 
theory  of  germ-regions  for  the  formation  of  organs  found 
support  in  observations  made  on  many  other  animals,  and  it 
was  discovered  that  even  in  the  ovum  the  so-called  primordial 
axis  gave  rise  to  an  animal  and  a  vegetative  pole,  determining 
the  direction  in  which  the  future  embryo  was  to  develop. 
Embryology  had  therefore  again  taken  an  appreciable  turn 
in  the  direction  of  the  preformation  theory. 

But  in  1883  there  was  an  apparent  reversion  to  epigenesis, 
in  consequence  of  the  experiments  made  by  Edward  Pfliiger, 
with  a  view  to  determining  the  influence  of  gravitation  upon 

in  the  imagination,  but  so  is  also  the  ontogenetic  moncrula  stage  in  the  develop- 
ment of  the  human  embryo.  For  a  criticism  of  Haeckel's  pedigree  of  man 
see  Chapter  XL 

1  Unsere  Kdrj)erform  und  das  physiologische  Problem  ihrer  Entstehung,  Leipzig, 

2  Wilson  suggests  '  Germinal  Localisation '  as  a  name  for  this  theory. — 
Translator's  Note. 


the  development  of  frogs'  eggs.  To  these  experiments  we  owe 
Pfliiger's  principle  of  the  isotropy  of  the  egg-plasm,  according 
to~~~which  all  the  protoplasmic"  constituents  of  the  egg  are 
collectively  of  equal  value  with  regard  to  the  formation  of  the 
organs  in  the  embryo.  Pfliiger  put  frogs'  eggs  in  what  he  called 
a  position  of  constraint,  so  that  the  egg  was  prevented  from 
turning  round  in  its  gelatinous  envelope,  owing  to  defective 
swelling  of  the  latter.  Under  normal  circumstances  the 
animal  half  of  the  frog's  egg,  which  consists  of  lighter  sub- 
stances and  contains  black  pigment,  always  is  uppermost, 
whilst  the  pale  yellow  vegetative  pole  is  underneath.  If, 
however,  the  egg  is  prevented  from  turning,  the  axis  of  the  egg 
can  be  made  to  form  any  desired  angle  with  the  vertical.  Even 
in  this  case  the  first  cleavage-plane  of  the  egg  as  it  develops 
will  always  be  vertical.  This  might  lead  us  to  believe  that 
gravitation  alone  determined  the  arrangement  of  the  parts 
of  the  embryo,  and  that  it  was  a  matter  of  indifference 
which  part  of  the  egg  lay  above  or  below  at  the  beginning 
of  cleavage. 

The  conclusions,  which  Pfliiger  deduced  from  this  fact  in 
favour  of  the  isotropy  of  egg-plasm,  proved,  however,  not  to 
be  tenable.  Wilhelm  Koux  and  Oskar  Hertwig  soon  suggested 
that  the  dependence  of  the  evolution  of  the  frog's  egg  upon 
gravitation  was  only  a  consequence  of  the  unequal  specific 
gravity  of  its  parts.  In  the  eggs  placed  in  abnormal  positions 
the  egg  envelope  was  prevented  from  turning,  but  the_rearrange- 
Tnopj^pf  f]]^  substances  within  the  egg  was  unaffectejj.  Born 
proved  this  by  experiments  of  his  own. 

In  order  to  disprove  Pfliiger's  theory  of  the  importance  of 
gravitation  in  directing  the  development  of  the  embryo, 
Koux  placed  some  frogs'  eggs,  already  developing,  on  a  disc 
that  rotated  vertically,  so  that  their  position  with  regard  to 
gravitation  was  constantly  changing.  In  spite  of  this,  their 
development  was  normal  both  as  to  time  and  manner.  Yet, 
as  Kathariner  has  recently  pointed  out,1  in  his  clinostatic 
experiments  Koux  had  replaced  the  force  of  gravitation  by 

1  Uber  die  bedingte  Unabhangigkeit  des  polar  differenzierten  Eis  von  der 
Schwerkraft '  (Archiv  fiir  Entwicklungsmechanik,  XII,  1901,  Part  4,  pp.  597- 
609) ;  '  Weitere  Versuche  liber  die  Selbstdifferenzierung  des  Froscheis ' 
(ibid.  XIV,  1902,  Parts  1  and  2,  pp.  289-299)  ;  '  Schwerkraftwirkung  oder 
Selbstdifferenzierung  ?  '  (ibid.  XVIII,  1904,  Part  3,  pp.  404-414). 


another  force,  viz.  the  centrifugal ;  and  consequently  it  was 
still  not  certain  that  the  development  of  the  egg  was  completely 
independent  of  an  external  directive  force. 

In  order  to  settle  this  point,  Kathariner  had  recourse  to 
another  method.  He  kept  the  fertilised  frogs'  eggs  in  constant 
rotation  by  means  of  a  stream  of  water.  Even  then  they 
developed  in  a  perfectly  normal  way,  although  somewhat 
more  slowly  than  usual.  These  experiments  have  proved 
conclusively  that  the  reasons  for  the  specific  development  jof 
a  frog's  egg  into  a  frog  are  in  the  egg  itself,  and  cannot  be  found 
in  any  external  influences.  The  development  of  the  egg 
depends  on  self-differentiation,  as  Koux  declared.  We  must 
regard  as  disproved,  once  for  all,  the  theory  which  Pfluger 
enunciated  as  follows,  in  support  of  epigenesis  :  '  I  am  of 
opinion  that  the  fertilised  ovum  no  more  bears  an  essential 
relation  to  the  subsequent  organisation  of  an  animal,  than  the 
snowflakes  do  to  the  size  and  shape  of  the  avalanche  to  which 
they  contribute  :  the  fact  that  out  of  a  germ  the  same  thing 
is  always  produced  is  due  to  its  being  always  subjected  to 
the  same  external  conditions.' 


When  we  find  scientific  men  like  Oskar  Hertwig,1  who  are  not 
far  from  being  vitalists,  still  feeling  bound  to  ascribe  to  external 
factors,  such  as  heat,  the  rank  of  causes  of  specific  development, 
we  must  believe  that  this  is  due  to  a  confusion  of  the  general 
conditions  of  development  with  its  particular  causes.  We  have 
many  external  means  of  accelerating  or  retarding  development, 
and  of  making  it  follow  a  normal  or  an  abnormal  course,  but 
we  are  never  able  to  alter  the  laws  of  specific  development,  for 
instance  in  the  frog's  egg.  If,  therefore,  such  an  egg  invariably 
produces  a  frog,  it  does  so  through  some  self-differentiation  in 
the  fertilised  ovum. 

If  we  regard  the  egg  with  its  capacity  for  development 
as  a  ivhole,  the  question  whether  preformation  or  epigenesis 
controls  its  action  is  therefore  already  answered  in  favour  of 

1  Die  Zelle  und  die  Gewebe,  II,  1898.  Cf.  my  remarks  on  0.  Hertwig's 
opinions  on  p.  220. 


preformation  ;  there  are  in  the  egg  some  dormant  tendencies 
which  underlie  its  specific  development.  But  this  is  not  a 
complete  solution  of  the  problem  of  determination. 

We  have  to  answer  another  and  a  much  more  difficult 
question:  'In  what  relation  do  the  individual  parts  of  the 
fertilised  ovum  stand  to  one  another  ?  Is  their  development 
fully  independent,  based  on  self -differentiation,  or  is  it  in  a 
state  of  regular  dependence  upon  the  other  parts  of  the  egg, 
and  based,  therefore,  on  a  dependent  differentiation  ?  ' 

.  I  have  already  discussed  Pfluger's  theory  of  the  isotropy 
of  egg-plasm,  according  to  which  all  parts  of  the  egg  are  quite 
uniform  in  material  and  in  their  influence  on  the  development 
of  the  various  organs  of  the  embryo  (see  p.  217).  This 
theory  must  be  given  up,  for,  as  Eoux  pointed  out,  even  before 
cleavage  begins,  the  median  plane  of  the  future  embryo  is 
determined  by  the  position  of  the  cleavage-nucleus  in  copulation, 
i.e.  by  the  course  taken  by  the  male  pronucleus  in  order  to 
unite  with  the  female  pronucleus,  and  so  form  the  cleavage 
nucleus  of  the  fertilised  egg.  Kecent  microscopical  research 
has  revealed  the  regular  distribution  of  the  chromatin  of  the 
cleavage-nucleus  to  the  daughter-cells  of  the  egg,  and  this 
distribution  introduces  the  development  of  the  embryo.  We 
must  therefore  ascribe  to  the  chromosomes  of  the  nuclei  an 
important  part  in  determining  the  formation  of  the  organs  in 
the  embryo.  This  consideration  gives  support  to  Koux  and 
Weismann's  theory  of  nuclear  regions  for  the  formation  of 
organs.  Here  too,  therefore,  the  theory  of  preformation  seems 
to  prevail  over  epigenesis. 

In  fact,  epigenesis  seems  almost  hopelessly  weak  as  a  theory, 
if  we  take  into  account  only  those  epigenetic  opinions  which 
are  based  on  mechanics,  and  aim  at  accounting  for  the  whole 
development  of  the  embryo  merely  by  the  attraction  and 
pressure  of  the  cleavage-spheres.  But  the  chief  supporters  of 
epigenesis — men  like  Oskar  Hertwig  and  Hans  Driesch — 
are  by  no  means  adherents  of  the  theory  of  mechanism  in  the 
ordinary  sense  of  the  word.  Oskar  Hertwig's  views  on  the 
subject  of  organic  development  have  much  in  common  with 
vitalism ;  he  has  expressed  them  in  his  earlier  works,  but  a 
concise  statement  of  them  may  be  found  in  his  *  Allgemeine 
Biologie,'  1906,  which  is  practically  a  new  edition  of  his  previous 


textbook  '  Die  Zelle  und  die  Gewebe  '  ('  The  Cell  and  the 
Tissues  ')  published  in  1898. 

In  discussing  the  various  internal  and  external  causes  of 
development  (pp.  132-140),  he  says  that  both  factors  must 
co-operate  in  every  process  of  development ;  but,  as  he  thinks 
the  internal  causes  (or  tendencies  to  development)  always 
form  tEe  basis  for  the  action  of  the  external  influences,  it  is 
impossible  to  say  that  he  gives  a  purely  mechanical  explanation 
of  the  process  of  development.  On  the  contrary  (pp.  141,  &c.), 
he  expressly  emphasises  the  '  very  important  differences 
existing  between  machines  and  organisms,  between  what  is 
mechanical  and  what  is  organic.'  In  his  '  Allgemeine  Biologie  ' 
he  devotes  only  two  chapters  (xx  and  xxi)  to  the  external 
factors  of  organic  development,  but  no  less  than  four  chapters 
(xxii-xxv)  to  the  internal  factors,  and  ascribes  to  them  the 
chief  importance,  especially  in  the  case  of  animals  (p.  508). 
He  expresses  himself  as  a  vitalist  in  speaking  of  the  various 
stages  of  the  process  of  development,  and  says  (p.  519)  :  *  The 
form  at  any  given  moment  appears  to  be  in  many  respects  a 
function  ot  the  growth  of  the_organic  substance  ;  its  persistence 
is  subject  to  definite  conditions  ;  and  as  they  change  in  con- 
sequence of  advancing  growth,  they  effect  a  modification, 
adapted  to  the  purpose  in  view,  in  the  form  of  the  substance, 
which  is  capable  of  reacting  under  their  influence.' 

At  the  close  of  this  chapter  I  shall  recur  to  Oskar  Hertwig's 
attitude  towards  vitalism.  In  1898  he  felt  bound  to  ascribe  to 
external  mechanical  causes  l  a  direct  formative  influence  upon 
the  process  of  development  in  many  cases,  but  in  1906  he 
modified  this  opinion  considerably.  His  earlier  views  were 
challenged  by  0.  L.  Zur  Strassen  in  a  lecture  delivered  on 
June  3,  1898,  at  the  eighth  meeting  of  the  German  Zoological 
Association  at  Heidelberg.2 

According  to  0.  Hertwig,  the  division  of  the  fertilised  ovum 
into  cells  of  equal  size  and  similar  structure  is  effected  by  the 
vitelline  contents  of  the  cells  and  the  external  shape  of  the 
cleavage-spheres  (blastomeres).  He  thinks  that  the  delicate 
mechanism  of  mitotic  karyokinesis,  in  which  the  egg  changes 
into  the  groups  of  cells  in  the  embryo,  is  the  cause  of 
cell-division  as  such,  but  not  of  the  differentiation  of  these  cells 

1  Die  Zelle  und  die  Gewebe,  II. 

2  '  tfber  das  Wesen  der  tierischen  Formbildung  '  (VerJiandl,  pp  142-156). 


to  form  organs  and  tissues,  although  the  two  processes  are 
connected.  Hertwig  attempts  to  account  for  unequal  cell- 
division  by  means  of  the  mechanical  influence  of  the  yolk 
contained  in  the  egg,  which,  he  thinks,  causes  the  daughter- 
cells  to  be  of  different  sizes.  If  more  deuteroplasm  is  accu- 
mulated at  one  pole  of  the  egg  than  at  the  other,  the  nucleus 
of  the  egg-cell  is,  according  to  Hertwig,  mechanically  pushed  to 
the  opposite  pole,  and  the  result  is  the  division  of  the  egg  into 
two  cleavage-spheres  of  unequal  size. 

Eeasonable  as  this  may  sound,  the  rule  still  does  not 
universally  hold  good,  and  there  is  not  a  purely  mechanical 
regularity  in  the  process  of  cell-division.  There  are,  fof 
instance,  as  Zur  Strassen  points  out,  a  number  of  cases  (e.g.  in 
the  cleavage  of  the  egg  of  the  maw- worm,  Ascaris)  where  the 
actual  process  is  the  direct  reverse  of  that  required  by  Hert- 
wig's  'law.'  In  this  particular  egg,  when  the  first  cleavage- 
spindle  is  formed,  the  upper  part  of  the  plasm  is  pale  in  colour 
and  poor  in  yolk  ;  whilst  the  lower  part  is  rich  in  yolk.  Never- 
theless, after  the  cleavage  the  upper  daughter-cell  is  the  larger, 
and  the  lower  is  the  smaller,  in  spite  of  its  abundance  of  yolk. 

0.  Hertwig  attempted  to  give  a  very  simple  account  of 
the  uneven  rate  of  division  of  the  cleavage-spheres  by  means 
of  the  mechanical  action  of  the  yolk.  He  thought  that  cells 
containing  much  yolk  divided  more  slowly  than  those  contain- 
ing less,  because  the  yolk  offered  an  external  resistance  to  the 
cleavage  processes  of  the  protoplasm.  But  here,  too,  there  are 
facts  in  direct  opposition  to  Hertwig's  mechanical  law.  Accord- 
ing to  Jennings,  in  the  development  of  the  Kotifer  Asplanclma 
and  of  many  other  species,  the  larger  cells,  that  are  rich  in  yolk, 
have  a  decided  tendency  to  divide  more  quickly  than  the 
smaller  cells,  that  are  poor  in  yolk. 

Purely  mechanical  factors  must  by  their  very  nature  always 
act  in  the  same  way,  and  these  '  exceptions  '  to  Hertwig's 
mechanical  laws  show  that  the  laws,  even  where  they  are 
apparently  observed,  are  not  purely  mechanical,  but  a  vital 
conformity  to  law  underlies  them,  controlling  and  regulating 
tneaction  of  the  mechanical  factors. 

Of  still  greater  importance  for  the  decision  of  the  question 
whether  the  development  of  the  organism  can  be  accounted 
for  on  purely  mechanical  grounds,  is  the  regular  direction  in 
which  the  cells  of  the  embryo  divide,  for  all  growth  in  a  definite 


direction  is  accompanied  by  a  corresponding  formation  of  the 
nuclear  figures  in  the  processes  of  mitotic  division,  and  there- 
fore the  series  of  cleavage  stages  in  the  developing  embryo  is 
based  primarily  upon  that  definite  direction  of  division.  If 
it  were  possible  to  find  a  purely  mechanical  principle  to  account 
for  this,  it  would  go  far  towards  enabling  us  to  explain  the 
processes  of  development  on  mechanical  lines.  Oskar  Hertwig 
thought  that  he  had  discovered  a  principle  of  this  kind,  and 
enunciated  the  following  '  law '  regarding  it :  '  The  division  - 
spindle  of  the  cell  is,  in  the  case  of  non-spherical  cells,  placed 
in  the  direction  of  the  largest  mass  of  protoplasm,  i.e.  in  the 
longest  axis  of  the  cell.' 

From  the  purely  mechanical  point  of  view  this  is  quite 
natural,  and  there  are  in  fact  many  cases  of  agreement  with 
this  law — but  there  are,  on  the  other  hand,  a  great  many  other 
facts  that  contradict  it. 

As  Zur  Strassen  points  out,  it  is  easy  to  bring  forward  an 
overpowering  number  of  instances  in  which  the  division- 
spindle  does  not  follow  the  longest  axis  of  the  cell,  which  would 
be  a  convenient  and  natural  arrangement  from  the  mechanical 
point  of  view,  but  it  follows  a  shorter  axis,  often  the  shortest 
possible,  so  that  it  seems  to  challenge  the  greatest  pressure 
instead  of  avoiding  it,  as  it  should  do,  if  Hertwig's  mechanical 
theory  were  correct.  This  occurs  in  all  cylindrical  epithelia 
and  also  in  very  many  of  the  early  blastula  stages  of  various 

With  regard  to  the  cleavage  stages  of  the  embryo,  it  has 
been  conclusively  shown  by  Jennings  in  the  case  of  a  Kotifer, 
Asplanchna,  by  Conklin  in  the  case  of  a  snail,  Crepidula,  by 
Bergh  in  various  Crustacea,  and  by  Sobotta  in  the  lancet 
fish,  Amphioxus,  that  there  is  no  such  thing  as  a  direct  in- 
fluence of  the  shape  of  the  cell  upon  the  direction  of  the  spindle 
that  is  easily  explicable  on  mechanical  lines.  There  is  therefore 
no  justification  for  Hertwig's  '  mechanical  law,'  as  stated 

1  By  the  blastula  stage  we  understand  the  first  development  of  the  embryo, 
in  which  the  ectoderm  is  formed  as  a  hollow  sphere  consisting  of  one  layer 
of  cells.  The  next  is  the  gastrula  stage,  in  which,  by  means  of  invagination 
of  part  of  the  blastula,  the  intestine  is  formed  and  the  entoderm  begins  to 
grow.  Between  ectoderm  and  entoderm  there  is  formed  subsequently  a 
third  layer  of  cells,  called  the  mesoderm. 


Still  less  is  there  any  justification  for  a  theory  propounded 
by  J.  Loeb,  an  American.  He  thinks  that  the  regular  inter- 
action of  the  parts  of  the  embryo  depends  upon  the  mechanical 
pressure  exercised  upon  one  another  by  the  crowded  cleavage- 
spheres,  forcing  them  by  merely  external  means  to  assume 
a  definite  geometrical  form.  Such  crude  attempts  at  explain- 
ing facts  on  mechanical  lines  are  almost  as  unsuccessful  in 
embryology  as  in  animal  psychology.1 

Zur  Strassen  has  arrived  at  the  following  conclusion  : — • 
'  That  the  cell  in  its  living  plasm  contains  mechanisms  enabling 

FIG.  27. 
sp  =  spindle. 

it  independently  to  discover  and  adopt  a  definite  direction  in 
division,  corresponding  to  the  aim  of  its  ontogeny.' 

He  proved  this  by  experimenting  with  the  eggs  of  the 
maw-worm,  Ascaris.  The  second  cleavage-division  affords 
a  classical  instance  of  the  formation  of  the  spindle  (sp)  in  the 
shortest  axis  of  the  cell  (fig.  27). 

If  there  were  only  purely  mechanical  causes  forcing  the 
protoplasm  to  set  the  spindle  in  this  position,  it  ought  to  be 
easy  to  induce  the  lower  cell,  which  is  subject  to  greater 
pressure  than  the  upper  (see  fig.  27),  to  develop  its  spindle 
on  its  longest  axis,  when  the  pressure  is  removed.  In  order 
to  effect  this,  Zur  Strassen  rolled  the  eggs  to  and  fro  under  a 
glass  until  they  were  no  longer  spherical,  but  of  a  long  oval 

1  On  the  latter  see  the  author's  article  '  Zur  mechanischen  Instinkttheorie ' 
(Stimmen  aus  Maria-Loach,  LX,  1901,  parts  2  and  3).  Also  Instinkt  und 
Intelligenz  im  Tierreich,  1905,  chapter  viii.  A  criticism  of  Loeb's  chemico- 
physical  theory  of  fertilisation  may  be  found  on  pp.  147,  &c. 



shape,  and  thus  the  two  cleavage-cells  had  room  enough  to 
develop  their  spindles  in  the  longest  diameter.  But  they  did 
not  do  so ;  in  the  lower  cell  also  the  spindle  retained  its  normal 
position,  although  it  was  in  the  shortest  axis  of  the  cell.  Similar 
observations  were  made  by  Zur  Strassen  at  the  two-celled 
stage  of  the  giant  eggs  of  Ascaris,  which  have  a  long,  oval 
shape,  and  their  cleavage-spheres  are  so  far  from  being  subject 
to  any  mechanical  pressure  that  they  float  freely  within  the 

FIG.  28. 

covering  of  the  egg,  and  touch  one  another  at  one  point  only. 
Yet  even  in  this  case  the  two  cells  developed  their  spindles 
in  the  shortest  axis  (fig.  28). 

These  experiments  in  embryology  lead  us  chiefly  to  the 
negative  result,  that  the  rj^echanicana^ws^idLdQWjQLby_Hertwig, 
Loeb,  and  others  arejnag^rate,  anH^supply  no  causal  explana- 
tion of  the  processes  we  are  discussing.  Zur  Strassen  thought 
that  his  experiments  justified  the  positive  conclusion :  '  That 
the  cell,  when  ready  to  divide,  contains  most  delicate  mechan- 
isms which  determine  the  moment  when  mitosis  shall  take 
place,  the  direction  of  the  spindles,  and  the  comparative  size 
of  the  products.  This  really  seems  as  if  the  cleavage-cell 
possesses  an  unerring  instinct  directing  the  process  of  cleavage.' 


Therefore  not  only  do  the  causes  determining  the  specific 
development  reside  in  the  egg  itself,  but  the  interaction  of 
the  various  parts  of  the  egg,  as  it  develops,  is  controlled  by  a 
teleological  law,  which  directs  the  mechanical  factors  towards 
the  aim  of  the  embryonic  development. 

This  has  brought  us  at  least  somewhat  nearer  to  a  solution 
of  the  problem  of  determination,  but  we  have  still  not  decided 
whether  preformation  or  epigenesis  underlies  the  whole  process 
of  development.  Weismann,  the  extreme  supporter  of  the 
theory  of  preformation,  says  that  ontogeny  can  be  explained 
only  by  evolution,  and  not  by  epigenesis.1 

Oskar  Hertwig,  on  the  contrary,  asserts  : 2  '  The  develop- 
ment of  a  living  creature  is  by  no  means  a  piece  of  mosaic 
work,  but  all  the  various  parts  develop  always  in  relation  to 
one  another,  or  the  development  of  any  one  part  is  always 
dependent  upon  the  development  of  the  whole.' 

Here,  as  in  every  case  where  scientists  hold  different  opinions, 
we  must  put  the  question  in  a  clear  and  definite  form,  in  order 
that  we  may  know  what  each  of  these  theories  involves. 

We  shall  therefore  ask  with  Korschelt  and  Heider : 3  '  Are 
there  present  in  the  egg,  when  it  begins  to  develop,  any  special, 
independent  Anlagen  or  fundaments,  which  develop  quite 
apart  from  the  other  portions  of  the  egg  and  become  definite 
formations  in  the  embryo  ?  And,  if  there  are  such  Anlagen, 
how  have  they  come  into  existence  ?  Can  other  Anlagen  of  a 
similar  kind  arise  later  ? 

'  Or :  Do  the  various  formations  in  the  embryo  never 
develop  independently  ?  Are  they  always  dependent  upon 
the  other  parts  of  it  ?  In  this  case  we  should  have  to  acknow- 
ledge the  existence  of  a  constant,  mysterious  influence  exercised 
by  the  whole  upon  its  several  parts. 

*  Or  :  Do  both  methods  of  formation,  the  dependent  and 
the  independent,  participate  in  the  development  of  the  embryo  ? 
and,  if  so,  to  what  extent  ?  ' 

In  the  first  case,  if  preformation  alone  controls  development, 

1  Das  Keimplasma,  Jena,  1892,  p.  184.     In  his  recent  lectures  on  the 
Evolution  Theory,  1902,  Weismann  still  maintains  a  decidedly  preformistic 
attitude,  although  he  concedes  a  great  deal  more  to  epigenesis  than  he  did 

2  Alter e,  und  neuere  Entwicklungstheorien,  Berlin,  1892,  p.  29.     Cf.  also  his 
Allgemeine  Biologie,  p.  632. 

3  Lehrbuch  der  vergl  Entwicklungsgesch.,  Part  I,  Jena,  1902,  pp.  93-94. 



the  development  not  merely  of  the  egg  as  a  whole,  but  of  each 
separate  organ  in  the  future  creature  would  depend  upon  self- 
differentiation  ;  it  would  be  mosaic  work,  and  nothing  else. 

In  the  second  case,  if  epigenesis  alone  controls  development, 
the  whole  ontogeny  of  the  organism  would  be  based  upon 
dependent  differentiation,  upon  which  the  idea  of  the  whole 
would  be  impressed. 

In  the  third  case,  we  should  have  to  trace  development 
partly  to  preformation  and  partly  to  epigenesis,  working 
together  harmoniously  to  produce  the  due  result.  We  might 
then  follow  Driesch  in  describing  the  ontogeny  of  the  individual 
as  an  epigenetic  evolution.  As  we  shall  see  presently,  this 
third  alternative  is  the  best,  and  comes  closest  to  the  truth. 

The  well-known  saying,  '  What  suits  one  does  not  suit 
another,'  is  applicable  not  only  to  the  circumstances  of  human 
lije,  but  to  the  phenomena  occurring  in  the  development 
of  living  beings.  In  different  kinds  of  eggs,  and  in  different 
stages  of  the  development  of  one  and  the  same  organism, 
intrinsic  and  dependent  differentiation  act  very  variously. 
We  must  therefore  follow  Korschelt  and  Heider,  and  examine 
the  individual  cases  and  the  embryological  experiments  of 
modern  research.  Before  doing  so,  however,  I  ought  to 
explain  some  expressions  introduced  mostly  by  Hans  Driesch, 
the  most  consistent  advocate  of  epigenesis.  In  spite  of  their 
learned  sound  they  are  all  quite  simple. 

Driesch  distinguishes  the  prospective  value  and  the  'prospec- 
tive potency  of  a  cell  or  a  cleavage-segment,  in  the  course  of  the 
development  of  an  individual  organism.  By  prospective  value 
he  understands  the  real  destiny  of  the  cell,  by  prospective 
potency  its  possible  destiny.  We  may  therefore  call  prospective 
value  also  destiny  in  development,  and  prospective  potency 
possibility  in  development.  We  shall  understand  the  dis- 
tinction better,  if  we  consider  something  analogous  in  human 
life.  Let  us  imagine  a  boy  with  an  Anlage  for  being  a  tinker. 
If  the  circumstances  of  his  life  permit,  and  he  really  becomes 
a  tinker,  it  was  his  prospective  value  to  be  a  tinker.  But  the 
prospective  potency  of  the  same  boy  was  plainly  far  wider ; 
according  to  his  natural  disposition  he  might  eventually  become 
a  knife-grinder  or  a  schoolmaster,  a  gunner  or  an  author.  Now, 
the  prospective  potency  of  a  cell  comprises  all  that  it  is  possible 


for  it  to  develop  into,  or  the  sum  of  the  dispositions  that  it 
contains,  of  which,  however,  only  one  or  very  few  can  ever 
be  set  in  action  in  the  process  of  development  ;  these  latter 
represent  the  prospective  value  of  the  cell  and  its  descendants. 
According  to  Brauer,  any  cleavage-sphere  of  the  freshwater 
polypus  Hydra  has  the  power  to  produce  ectoderm  and  entoderm 
cells.  But  the  ectoderm  cells  of  later  stages  in  the  develop- 
ment of  the  same  animal  have  lost  the  power  to  produce 
entoderm  cells.  Thus  in  course  of  ontogeny  (or  the  develop- 
ment of  the  individual)  the  prospective  potency  of  the  cells  of 
Hydra  suffers  limitation.  In  general  we  may  lay  down  this 
principle  :  The  prospective  potency  of  a  cell  is  more  lipnitfifj_ 
in  higher  organisms  than  in  lower,  and  in  the  more  advanced 
stages  of  ontogeny  than  in  the  earlier  ;  it  may  even  cease  to 
exist,  and  we  have  an  instance  of  this  in  the  cormfied  cells 

Whoever  accepts  the  theory  of  prospective  potency  has 
practically  recognised  the  truth  of  epigenesis,  for  whenever 
we  speak  of  the  possibility  of  development,  we  mean  that  cells, 
or  groups  of  cells,  which  were  originally  designed  to  make  up 
some  definite  formation,  may,  under  certain  circumstances,  take 
another  direction  and  serve  another  end.  This  process  of 
transformation  has  been  called  redifferentiation  or  redeter- 
mination.  In  such  processes  the  influence  of  the  whole  in  some 
mysterious  way  is  brought  to  bear  upon  the  parts  of  the 
organism,  and  through  this  influence  they  co-operate,  so  as  to 
develop  a  creature  capable  of  life.  All  processes  of  develop- 
ment that  have  this  character  are  known  as  regulatory,  or  as 
organic  regulations,  these  being  the  names  used  by  Driesch.1 

Closely  connected  with  Driesch's  theory  of  prospective 
potency  or  possibility  of  development  in  cells  is  his  other  idea 
of  the  equipotential  system.  Such  a  system  is  formed  by  a 
group  of  cells,  each  of  which  possesses  the  same  potency. 
Driesch  subdivides  these  systems  into  determined  equipotential 
systems  and  undetermined  or  harmonious  equipotential  systems. 
In  the  former,  the  number  of  things  that  can  possibly  be  made 
from  the  group  of  cells  under  consideration  is  strictly  limited. 

1  I  need  not  discuss  the  further  distinction,  also  due  to  Driesch,  between 
primary  and  secondary  regulations,  primary  and  secondary  prospective 
potencies,  &c. 

Q  2 


For  instance,  from  any  transverse  section  of  a  willow  branch 
either  a  shoot  or  a  root  may  be  formed,  but  the  prospective 
potencies  of  the  cells  of  the  piece  of  willow  are  limited  to  these 
two  things.  But  in  the  harmonious  equipotential  systems 
any  one  element  can  assume  any  part,  and  so  the  number 
of  possible  developments  is  very  great.  Each  portion  of 
such  a  system  can  likewise  accomplish  a  whole  complicated 
process  of  formation  ;  which  form  it  will  assume  depends  upon 
the  position  borne  by  the  part  with  regard  to  the  whole,  for 
all  parts  are  harmoniously  subordinated  to  the  whole,  whence 
the  system  has  its  name  of  '  harmonious  equipotential.'  Thus, 
for  instance,  each  of  the  cells  of  the  thirty-two  cell  cleavage 
stage  in  the  egg  of  the  sea-urchin  is  not  only  able  to  form  the 
-^  part  of  the  embryo,  which  it  is  its  proper  function  to 
form,  but,  if  the  32  cells  are  artificially  separated  from  one 
another,  each  of  them  is  capable  of  developing  into  a  very  small, 
but  still  complete,  sea-urchin  larva. 


The  scale  seems  now  to  be  turning  again  in  the  direction  of 
epigenesis,  but  before  pronouncing  a  final  decision,  and  deduc- 
ing conclusions  for  or  against  the  theories  of  mechanism  and 
vitalism  respectively,  we  must  briefly  consider  the  various 
groups  of  animals  on  which  embryological  experiments  have 
chiefly  been  made. 

We  must  mention  first  the  experiments  on  the  eggs  of 
Amphibia,  begun  by  W.  Boux  in  1883.  With  a  heated  needle 
he  pricked  one  of -the  first  pair  of  cleavage-spheres  of  a  frog's 
£g&  and  so  killed  it.  The  other  half,  that  remained  uninjured, 
developed  exactly  as  if  the  destroyed  portion  had  remained 
alive,  but,  as  the  latter  was  incapable  of  development,  the 
result  of  the  experiment  was  the  production  of  a  half-embryo 
(hemiembryo  later •alis),  i.e.  a  future  frog  cut  in  two  lengthwise. 
Eoux  succeeded  also  in  destroying  a  cleavage-sphere  at  the 
four-cell  stage,  and  then  a  three-quarter  embryo  was  produced. 

These  results  justified  the  conclusion  that  under  ordinary 
circumstances  the  two  cleavage-spheres  of  the  two-cell  stage 
of  development  in  the  embryo  frog  contain  the  rudiments  of 


the  right  and  left  half  of  the  future  frog  respectively,  and  these 
rudiments  have  the  power  to  develop  independently  of  one 
another.  In  the  same  way,  each  quarter  at  the  four-cell  stage 
seemed  able  to  produce  a  corresponding  quarter  of  a  frog, 
without  being  affected  by  the  remaining  three  quarters. 

Eoux  formulated  his  results  as  follows  :  '  Normal  develop- 
ment is  from  the  outset  a  system  of  definitely  directed  processes; 
it  is  intimately  connected  with  the  chief  directions  in  which  the 
embryo  develops,  so  that  the  first  four  cleavage-cells  do  not 
merely  each  occupy  the  position  of  a  definite  quarter  of  the 
embryo,  but  are  capable  of  producing  each  its  proper  quarter 
independently.'  '  The  development  of  the  frog  gastrula  and 
of  the  embryo  resulting  immediately  from  it,  is,  from  the  second 
cleavage  onwards,  a  mosaic,  made  up  of  at  least  four  vertical 
pieces  developing  independently.' 

The  development  of  the  frog's  egg  appeared,  therefore,  to 
obey  the  laws  of  preformation  and  intrinsic  differentiation,  not 
those  of  epigenesis  and  dependent  differentiation,  but  obviously 
it  was  not  permissible  to  regard  this  result  as  applicable  gene- 
rally to  the  ontogeny  of  other  organisms.  Even  in  the  case  of 
the  frog,  Roux  observed  subsequently  that  his  half-embryos 
afterwards  grew  into  complete  ones,  as  the  missing  half  of 
the  body  was  supplied  by  the  existent  half,  by  means  of  the 
materials  from  the  cleavage-sphere  which  was  injured  by  the 
operation.  A  process  of  redifferentiation  set  in,  changing 
the  half  into  a  whole  embryo — a  regulation  which  unmistakably 
aimed  at  the  production  of  a  complete  creature,  capable 
of  life.  All  the  theories  of  preformation  and  mechanism  fail 
to  account  for  this  phenomenon. """ 

Oskar  Hertwig  repeated  Roux's  experiments  on  frogs'  eggs, 
but  came  to~quite  different  results.  He  observed  that  when- 
ever he  destroyed  one  of  the  first  pair  of  cleavage-spheres, 
with  one  solitary  exception,  the  uninjured  half  did  not  produce 
a  half-embryo,  but  a  complete  embryo  of  half  the  normal  size. 
Here,  therefore,  we  find  no  trace  of  mosaic  work,  but  only 
confirmation  of  the  laws  of  dependent  differentiation,  which  is 
dominated  by  the  idea  of  the  whole. 

It  was  reserved  for  0.  Schulze  and  Th.  Morgan  to  give 
by  their  experiments  a  satisfactory  explanation  of  the  apparent 
discrepancy  between  the  results  at  which  Roux  and  Hertwig 


had  arrived,  whilst  employing  the  same  methods  on  the  same 

Whenever  Morgan  left  the  fro_gs'  eggs  after  the  operation 
in  their  natural  position,  i.e.  with  their  black  (animal)  pole 
upwards,  the^uninjured  halves  invariably  produced  half- 
embryos.  When  he  turned  them  round,  so  that  the  white 
(vegetative)  pole  was  uppermost,  as  a  rule  complete  embryos 
of  half  the  normal  size  were  developed.  In  the  former  case,  the 
original  arrangement  of  the  egg-substance  was  retained  in  the 
uninjured  blastomere,  which  continued  its  ordinary  course  of 
development,  and  only  turned  into  a  complete  embryo  by  later 
^differentiation.  In  the  latter  case,  on  the  contrary,  turning 
the  egg  round  altered  the  arrangement  of  its  contents  in  a  way 
which  led  directly  to  a  regulation  of  the  development  in  accor- 
dance with  the  design  of  the  whole.  In  neither  case  can  we 
dispense  with  a  principle  regulating  embryonic  development. 

From  the  above-mentioned  embryological  experiments, 
and  from  others  of  a  similar  nature,  we  may  conclude  that 
under  normal  circumstances  the  first  two  cleavage-cells  in  the 
frog's  egg  possess  a  different  prospective  value,  inasmuch 
as  they  form  each  one  symmetrical  half  of  the  embryo.  But 
their  prospective  potency  is  identical,  and  equivalent  to  that 
of  the  egg  before  cleavage,  for  each  half  can  produce  a  whole 
embryo.  The  same  is  true  of  the  four  blastomeres  at  the 
four-cell  cleavage  stage  of  the  frog's  egg.  Each  is  under 
normal  circumstances  designed  to  give  rise  only  to  a  definite 
quarter  of  a  frog,  but  if  they  are  separated,  each  can  produce  a 
complete,  though  very  diminutive  creature.  At  later  periods 
of  embryonic  development,  however,  frQm_the_eight-aell  stage 
onwards,  the  cleavage- cells  are  not  any^ Jonger  all  of  the  same 
value.  At  this^stage  the  fouFcehVof  the  animal  half  of  the 
ovum  can  produce  only  organs  of  the  animal  sphere,  and 
those  of  the  vegetative  half  only  organs  of  the  vegetative 
sphere.  The  prospective  potency  of  the  cleavage- cells  of  the 
Amphibian  egg  becomes  more  limited  and  restricted  as  develop- 
ment proceeds. 

We  come  now  to  experiments  on  the  eggs  of  Echinoderms. 
In  these,  as  in  the  eggs  of  Amphibia,  the  chief  axes  of  the 
embryo  are  probably  determined  before  the  beginning  of  the 
cleavage  process,  although  we  do  not  know  with  certainty 
on  what  material  and  structural  circumstances  this  pre- 


formation  depends.  In  the  Amphibian  egg  the  different 
colouring  of  the  two  poles  indicates  an  animal  and  a  vegetative 
half  of  the  egg,  but  in  the  Echinoderm  egg  no  such  difference 
in  the  egg-substance  is  perceptible. 

Among  the  eggs  of  Echinoderms,  those  of  the  sea-urchin 
are  particularly  well  suited  for  embryological  experiments, 
and  are  often  chosen  for  the  purpose.  In  them  it  is  possible 
to  separate  the  blastomeres  of  the  egg  undergoing  cleavage, 
not  only  by  means  of  needles  or  by  shaking  the  vessel  of  water 
containing  the  eggs,  but  the  blastomeres  can  be  isolated  much 
more  satisfactorily,  as  Curt  Herbst  was  the  first  to  discover, 
if  the  eggs  are  put  into  water  containing  no  lime.  The  absence 
o:T  lime  alone  suffices  to  induce  the  blastomeres  to  develop 
in  isolation ;  in  fact,  at  somewhat  advanced  stages  in  the 
development  of  the  embryo,  it  is  only  necessary  to  put  it 
into  water  containing  no  lime,  in  order  to  separate  the  cells 
from  one  another. 

The  capacity  for  regulation,  or  power  of  redifferentiation  in 
the  cleavage-spheres,  is  possessed  by  sea-urchins'  eggs  in  a 
very  unusual  degree,  and  has  led  to  true  triumphs  for  the 
theory  of  epigenesis.  In  the  eggs  of  Amphibia  only  the  first 
four  cleavage-cells  of  the  embryo,  if  separated  from  one  another, 
are  capable  of  producing  a  fresh,  complete  embryo  ;  but  in 
sea-urchins'  eggs  this  power  lasts  as  far  as  the  blastula  stage^ 
which,  according  to  Hans  Driesch's  very  careful  calculation, 
consists  of  j3QB_£fiUs.  Each  of  these  808  cells  is  equivalent  to 
all  the  rest,  as  far,  as  its  power  of  development  is  concerned. 
Driesch  used  a  fine  pair  of  scissors  to  cut  up  some  sea-urchin 
embryos  at  the  blastula  stage.  He  cut  them  in  all  directions,  / 
haphazard,  and  first  the  raw  edges  drew  together  and  closed 
the  wounds,  then  the  piece  cut  off  became  a  little  round  blastula, 
which  followed  the  normal  course  of  development  and  finally 
produced  a  perfect,  though  small,  larva  (Pluteus)  of  the  sea- 
urchin.  If  the  blastula  had  been  left  untouched,  and  had 
followed  the  usual  course  of  development,  the  cells  situated 
where  the  incisions  were  made  would  have  occupied  quite  a 
different  position  in  the  embryo,  and  would  have  served  to 
form  quite  different  tissues  ;  for  instance,  they  might  have 
formed  the  intestine  and  not  the  outer  skeleton  of  the  body. 
Driesch's  experiments  have  proved,  therefore,  that  in  the  sea- 
urchin  blastula  all  the  cells  are  still  equivalent  to  one  another 


with  regard  to  their  power  of  development ;  each  of  them  can 
occupy  any  position  and  discharge  any  function  in  the  formation 
of  the  organism.  All  the  cells  of  the  Echinus  blastula  are  alike 
in  their  prospective  potency,  and  what  each  cell  becomes, 
i.e.  its  prospective  value,  is  determined  by  its  position  in  the 
whole  blastula,  which  is  itself  already  determined  by  the 
direction  of  its  axes.  Driesch  has,  as  a  result  of  his  experi- 
ments, enunciated  the  statement :  '  The  prospective  value 
of  the  cell  is  a  function  of  its  position.' 

The  Echinus  blastula  is  a  beautiful  instance  of  a  harmonious, 
equipotential  system,  in  which  each  part  is  able  to  take  the 
place  of  any  other  part,  or  to  become  a  complete  embryo.  Just 
as  the  soul  of  man  is  wholly  in  every  part  of  his  body,  and  wholly 
in  the  entire  body,  so  is  the  power  of  organic  development 
in  this  case  present  wholly  in  every  part  of  the  embryo  and 
wholly  in  the  entire  embryo.  Without  a  principle  regulating 
its  development  and  controlling  the  mechanical  factors,  this 
wonderful  unity  in  multiplicity  would  be  inconceivable.  Only 
vitalism  can  offer  any  satisfactory  explanation  of  this  phe- 
nomenon ;  mechanics  cannot  account  for  it. 

The  further  the  development  of  the  organs  has  advanced  in 
the  Echinus  larva,  the  less  is  the  power  of  redifferentiation 
possessed  by  the  individual  cells.  In  this  case  too,  as  in  that 
of  the  development  of  the  embryo  frog,  the  prospective  potency 
of  each  cell  is  diminished  as  growth  goes  on,  although  in  the 
sea-urchin  it  remains  unrestricted  until  the  blastula  stage  is 
reached.  Driesch  remarks  that  the  organs  in  their  original 
Anlage  or  disposition  are  without  exception  the  result  of  depen- 
dent differentiation  in  the  widest  sense,  but  in  their  develop- 
ment they  show  intrinsic  or  self-differentiation  in  the  literal 
sense  of  the  word.  It  seems,  then,  that  here  too  epigenesis 
must  be  reconciled  with  preformation,  if  we  are  to  give  any 
complete  account  of  the  process  of  development. 

Let  us  now  refer  shortly  to  experiments  on  the  ova  of 
other  classes  of  animals. 

In  the  ova  of  Hydromedusae  (Polypi  and  Medusae)  the 
cleavage-spheres,  when  isolated,  behave  as  do  those  in  the 
ovum  of  the  Triton  among  Amphibia.  A  cleavage-sphere  after 
isolation  becomes  round,  and  forms  a  diminutive  whole, 
continuing  its  cleavage- divisions  and  resulting  finally  in  the 


formation  of  a  very  small,  but  otherwise  normal  larva.  Zoja 
bred  perfect  Hydroid  polypi  from  isolated  blastomeres  of  the 
two-  and  four-cell  stages,  but  only  larvae  (Planulae),  from  those 
of  the  eight-  and  sixteen-cell  stages,  and  these  larvae  had  no 
power  of  further  development.  Therefore,  we  have  here 
another  instance  of  restriction  of  the  prospective  potency  in 
the  cleavage-cells  of  the  embryo,  proportionate  to  the  advance 
in  its  development. 

A  comparison  between  these  embryological  experiments 
and  others,  made  on  the  eggs  of  Ctenophora  with  tentacles, 
will  show  what  great  diversities  can  exist  in  the  laws  governing 
the  development  of  closely  related  groups  of  animals.  In 
the  ova  of  the  Ctenophora  a  limitation  of  the  prospective 
potency^  of  the  individual  blastomeres  sets  in  very  early,  so 
that  we  are  reminded  of  the  mosaic  theory.  The  first  experi- 
ments were  made  by  Karl  Chun,  who  succeeded  in  shaking 
apart  the  two  blastomeres  resulting  from  the  first  cleavage  of 
the  ovum  of  tentacular  Ctenophores  and  in  breeding  from 
them  two  half -larvae,  each  possessing  four  ribs  instead  of  eight 
(the  normal  number),  and  having  only  half  the  usual  number 
of  other  organs  too.  Subsequent  research  has  confirmed 
Chun's  observations  on  all  essential  points,  and  we  may  say 
that  in  Ctenophores  the  first  two  cleavage-spheres  of  the 
fertilised  ovum  have  each  a  clearly  defined  prospective  potency ; 
each  can  produce  only  half  a  normal  organism,  whilst  among 
the  true  Medusae  belonging  to  the  same  subdivision  of  the 
animal  kingdom,  each  cell  at  the  sixteen-cell  stage  is  still 
capable  of  producing  a  complete  little  larva.  The  development 
of  the  first  pair  of  blastomeres  in  the  ovum  of  a  Ctenophore  is 
a  genuine  mosaic,  which  depends  on  self-differentiation,  each 
half  of  the  ovum  being  quite  independent  of  the  other  half. 
The  same  is  true  of  the  formation  of  the  fourth  and  eighth 
parts  of  the  embryo,  which  are  produced  by  subsequent 
cleavage-divisions.  Not  until  the  ectoderm  has  grown  over  the 
embryo  is  any  co-operation  and  reciprocal  action  perceptible 
between  the  fourth  and  eighth  parts. 

The  development  of  the  ribs  in  the  embryo  of  a  Ctenophore 
is  peculiarly  interesting.  All  who  have  made  experiments  on 
the  fertilised  ovum  of  Ctenophore  agree  in  believing  that  it 
can  produce  eight  ribs  and  no  more.  As  the  process  of 


cleavage  goes  on,  the  possibility  of  producing  them  is  so  far 
localised,  that  to  each  eighth  is  assigned  the  task  of  forming 
one  rib.  As  the  Anlagen  for  the  ribs  arise  from  the  little 
cleavage  -  spheres,  or  micromeres,  of  the  embryo,  which 
differentiate  themselves  from  the  large  cleavage-spheres,  or 
macromeres,  at  the  sixteen-cell  stage,  we  must  say  that  each 
of  the  eight  micromeres  possesses  the  Anlage  to  form  one  rib, 
and  its  development  is  therefore  a  real  intrinsic  differentiation. 

Although  there  is  no  connexion  between  Molluscs  and 
Ctenophores,  their  eggs  behave  in  the  same  way  during  the 
process  of  cleavage.  Isolated  blastomeres  continue  to  divide 
as  if  they  were  still  in  union  with  the  whole,  and  show  conse- 
quently partial  cleavage.  It  is  true  that  the  half-  or  quarter- 
embryos  thus  produced  do  not  correspond  exactly  to  a  half  or 
a  quarter  of  the  organism  under  observation,  but  they  become 
so  far  complete  as  to  be  capable  of  life,  the  ectoderm  covers 
them  abundantly,  and  there  are  some  attempts  at  forming  the 
velum  of  the  normal  larva.  But  it  has  proved  impossible  to 
breed  these  creatures  any  further  ;  they  died  in  every  case  at 
this  point.  The  development  of  the  Mollusc  ovum  depends, 
therefore,  essentially  upon  self-differentiation  of  the  individual 
blastomeres,  and  can  be  described  as  a  mosaic.  An  equally 
pronounced  mosaic  character  is  displayed  by  the  cleavage 
process  of  the  ovum  of  Annelida  and  Nematoda. 

Chabry's  experiments  on  the  egggjof  Ascidia  seemed  also 
to  support  the  mosaic  theory  and  preformation,  for  by 
separating  the  first  two  cleavage-spheres  half-larvae  were 
produced,  but  subsequent  experiments  made  by  Driesch 
and  Crampton  have  shown  that  these  eggs  resemble  in  this 
respect  those  of  many  of  the  Echinoderms,  for  instance  those 
of  a  sea-urchin  (Sphaerediinus).  Interference  with  the  cleavage- 
cells  and  their  isolation  cause  at  first  a  defective  cleavage, 
producing  only  part  of  an  embryo,  but  subsequently  readjust- 
ment sets  in,  and  the  part  develops  to  a  whole,  so  that  finally 
complete  blastulae,  gastrulae,  and  larvae  are  formed,  but  of 
reduced  size. 

In  the  eggs  of  Ctenophora,  Molluscs  and  many  worms 
there  is  only  a  very  slight  power  of  readjustment,  and  their 
development  appears  as  a  mosaic  work,  but  the  eggs  of  the 
bony  fishes  (Teleostei)  and  those  of  the  famous  Amphioxus 


in  regulatory  power  resemble  those  of  the  Echinoderms.  There 
is,  however,  in  the  eggs  of  the  Amphioxus  a  certain  tend- 
ency to  defective  cleavage,  i.e.  to  the  formation  of  imperfect 
embryos,  and  there  is  also  a  very  rapid  diminution  in  the 
power  of  redifferentiation  as  the  process  of  cleavage  goes  on. 
In  spite  of  this,  however,  at  least  in  the  early  stages  of 
cleavage,  dependent  differentiation  is  far  more  apparent  than 


We  have  now  completed  our  survey  of  the  embryological 
development  of  the  eggs  of  various  kinds  of  animals,  and  we 
may  pass  on  to  the  conclusions  to  be  deduced  from  it.  It  will 
tend  to  brevity  and  clearness  if  I  present  them  in  the  form 
of  questions. 

First :  '  Is  the  ontogeny  of  the  organism  based  upon 
independent  or  dependent  differentiation,  on  preformation 
or  epigenesis  ?  ' 

If  we  regard  the  fertilised  ovum  as  a  whole,  then  its  em- 
bryonic development  from  beginning  to  end  is  based  upon 
independent  differentiation,  and  consequently  upon  preforma- 
tion. But  if,  on  the  contrary,  we  take  into  account  the  relations 
to  one  another  of  the  individual  parts  of  the  egg  and  of  the 
embryo  to  be  produced  from  it,  the  answer  to  the  question  is : — 
Development  is  based  partly  on  intrinsic,  and  partly  on 
extrinsic  o^_dep_endent_^.fIerentiation.~Viewed  as  a  whole, 
the  process  of  development  appears  to  be  an  epigenetic  evolu- 
tion. Considered  in  detail,  in  the  ontogeny  of  living  organisms 
dependent  and  independent  differentiation  act  in  many  respects 
conjointly,  but  in  many  other  respects  quite  distinctly,  not 
only  in  the  eggs  of  various  animals,  but  in  the  stages  of  develop- 
ment in  the  same  embryo.  Sometimes  the  development  of 
the  parts  of  the  embryo  resembles  a^  mosaic  jwork,  in  which 
each  part  takes  its  form  irrespective  of  the  other  parts,  as  in 
the  Ctenophores.  Sometimes  it  is  more  like  a^harmonious 
equi^otential  system,  in  which  each  part  is  able  to  exchange 
its  role  with  every  other  part,  or  even  to  undertake  the 
duty  of  the  whole,  as  in  the  sea-urchin  blastula.  In  both 
cases,  however,  the  regular  course  of  the  various  phases  in 


development  is  controlled  by  the  idea  of  the  whole  that  is  to 
be  produced,  although  in  the  latter  case  the  idea  is  certainly 
clearer  and  more  definite  than  in  the  former. 

We  have  seen  that  at  the  beginning  of  embryonic  develop- 
ment, the  cleavage- cells  of  the  embryo  generally  display  a  far 
greater  power  of  readjustment  or  redifferentiation  than  they 
do  later,  and  thus  the  prospective  potency  of  the  individual 
cells  is  diminished  the  further  the  organs  of  the  new  creature 
develop.  From  this  point  of  view,  development  begins  with 
dependent  differentiation  and  ends  with  intrinsic  differentia- 
tion of  the  various  parts  of  the  embryo. 

Second :  '  What  connexion  is  there  between  the  nuclear 
substance  of  the  egg^ell  and  the  development  of.  the  embryo  ?  ' 

This  difficult  question  has  already  been  discussed  from  the 
standpoint  of  microscopical  morphology  in  Chapter  VI ;  we 
must  now  refer  to  it  shortly  on  its  embryological  side.  On 
this  subject  there  are  two  opinions  current,  in  direct  antagonism 
to  each  other.  According  to  one,  supported  chiefly  by  Wilhelm 
Eoux  and  August  Weismann,  the  chromatin  nuclear  substance 
of  the  fertilised  ovum  and  the  cleavage-cells  formed  from  it 
exercises  a  controlling  and  regulating  influence  over  the  pro- 
cesses of  development  By  means  of  what  Weismann  calls 
erbungleiche  Teilung,  or  differential  division,  the  chromosomes 
of  the  cell-nuclei,  which  are  the  material  bearers  of  heredity, 
are  distributed  in  different  ways  to  the  different  cells  of  the 
organism  that  is  to  be  produced,  and  thus  they  determine  the 
character  of  the  future  tissues  and  organs.  The  other  theory, 
however,  which  is  upheld  chiefly  by  Oskar  Hertwig  l  and  Hans 
Driesch,  denies  both  the  existence  and  the  necessity  of^any 
differential  division  of  the  chromosomes.  It  recognises  the 
facts  that  they  are  to  be  regarded  as  material  bearers  of  heredity, 
and  that  they  possess  a  certain  amount  of  individual  independ- 
ence, but  it  does  not  ascribe  to  them  so  great  a  determining 
importance  in  the  processes  of  development  as  the  former 
theory  assigns  to  them. 

Both  theories  find  support  in  significant  facts,  although 
there  are  other  facts  which  can  hardly  be  reconciled  with  them. 

The  theory  of  differential  division  stands,  perhaps,  in  more 
logical  connexion  with  the  processes  of  karyokinesis  that 

1  Allgemeine  Biologie,  pp.  356,  &c.,  454,  &c. 


have  been  observed  under  the  microscope  as  taking  place 
during  fertilisation.  These  show  us  not  merely  the  regular 
distribution  of  the  chromatin  substance  of  the  nuclei  of  the 
germ-cells  to  the  daughter-cells  of  the  embryo,  but  also  a 
division,  which  at  least  in  many  cases  seems  to  be  differential, 
as  the  future  germ-cells  and  the  future  somatic  cells  receive 
remarkably  unequal  amounts  of  chromatin.  Boveri  and 
other  scientists  have  shown  this  to  occur  in  the  egg  of  the 
maw-worm,  Ascaris  megalocephala  var.  bivalens,  and  Giardina 
has  observed  it  in  that  of  the  water-beetle,  Dytiscus.1 

The  theory  of  differential  division  may  find  support  also 
in  the  embryological  phenomena  already  described,  in  which 
the  development  of  the  embryo  is  controlled  chiefly  by  the 
self-differentiation  of  its  various  parts,  and  therefore  represents 
a  mosaic,  as,  for  instance,  in  the  Ctenophores.  Moreover  the 
fact  that,  as  the  development  of  the  embryo  advances,  the 
prospective  potency  of  its  cells  diminishes  and  becomes  more 
limited,  can  easily  be  explained  by  the  theory  of  differential 

But  against  this  theory  and  in  favour  of  erbgleiche  Teilung, 
or  integral  division,  there  are  many  other  facts  in  embryology 
which  have  been  carefully  observed  and  are  of  no  less  signifi- 
cance, the  chief  of  them  being  that  the  single  cells  of  the 
embryo  may  form  an  equipotential  system,  the  component 
parts  of  which  may  be  set  to  discharge  the  functions  of  any 
other  parts  or  even  of  the  whole.  When  the  sea-urchin  egg  is 
in  course  of  cleavage,  each  part  of  the  blastula,  cut  haphazard 
in  any  direction,  is  capable  of  becoming  a  complete  blastula 
able  to  develop  further.  This  fact  would  seem  to  justify  the 
conclusion  that  the  nuclear  substances  of  the  single  cells  in 
the  embryo  are  absolutely  equivalent  to  one  another,  and 
that  consequently  no  differential  division  can  have  taken 
place  at  the  cleavage  of  the  ovum.  Against  the  theory  of 
differential  division  is  the  further  fact  that  the  development 
of  the  special  Anlagen  for  the  future  organs  in  the  embryo 
is  based  chiefly  upon  dependent  differentiation,  whilst  self- 
differentiation  asserts  itself  more  in  subsequent  stages.  It 
appears,  therefore,  that,  if  we  leave  out  of  consideration  the 
very  early  differentiation  between  germ-cells  and  somatic  cells, 

1  Of.  p.  122  and  fig.  23,  p.  124  ;  also  p.  169. 


as  a  rule  only  an  integral  division  of  the  bearers  of  heredity 
takes  place  at  the  beginning  of  embryonic  development.  It  is 
possible  that  future  research  will  show  us  how  to  reconcile 
these  two  theories  of  integral  and  differential  division,  but  at 
present  they  are  involved  in  many  difficulties,  and  it  is  not 
easy  to  view  them  impartially. 

Of  far  greater  importance  than  this  purely  technical  ques- 
tion is  another,  which  is  concerned  with  the  philosophical 
solution  of  the  problem  of  life,  and  must  therefore  be  discussed 
more  fully. 

Third  :  '  Dp  mechanical  causes  suffice  to  afford  a  satisfac- 
tory explanation  of  the  processes  of  development,  orjtnust 
we  accept  aspecial  "  vital  "  law  to  account  for  them, — a  law 
goveTntngTEe^  chemico -physical  factors  of  development,  and 
directing  them  to  the  formation  of  an  organism  capable  of 
living  ?  '  In  other  words  :  '  In  attempting  to  offer  a  philo- 
sophical account  of  the  phenomena  of  embryonic  development 
must  we  profess  ourselves  adherents  of  the  "  machine  theory  " 
or  of  vitalism  ?  ' 

Vitalism  is  as  old  as  natural  philosophy  itself.  It  is  well 
known  that  the  scholastic  philosophers  adopted  special  formal 
principles  (entelechies)  as  the  actual  essential  forms  of  living 
matter,  in  order  to  account  for  the  phenomena  of  life. 

This  is  the  earliest  kind  of  vitalism,  but,  at  the  beginning 
of  the  nineteenth  century,  it  had  been  more  or  less  forgotten 
in  scientific  circles.  Liebig  and  other  chfmm'ata  thought  that 
they  must  assume  the  existence  of  a  SDecialkind  of  vital  force 
working  in  living  organisms,  over  and  above  mechanicaHorces. 
Towards  the  end  of  the  century  neovitalism  entered  upon  a 
new  stage,  approximating  to  the  vitalism  of  the  old  philo- 
sophers. Two  of  the  chief  advocates  of  neovitalism,  J.  Reinke, 
the  botanist,  and  Hans  Driesch,  the  zoologist,  do  not  regard 
the  principle  of  life  as  a  causa  efficiens  of  the  vital  processes, 
but  as  an  internal  formal  principle  of  the  living  organism.  We 
shall  recur  to  this  topic  later  (cf.  p.  243). 

The  machine  theory  was  the  outcome  of  the  great  success 
with  which  the  mechanical  view  of  nature  was  applied  to 
physics  and  chemistry  in  the  nineteenth  century,  but,  when  it 
is  closely  examined,  it  is  found  to  be  based  upon  a  one-sided 
overvaluation  of  the  importance  of  mechanics  in  explaining 


natural  phenomena,  and  it  cannot  hold  its  own  against  a 
thorough  criticism.  It  still  has  many  adherents,  for  old 
prejudices  die  hard.  Professor  Otto  Biitschli  defended  it 
against  the  supporters  of-neovitalism  at  the  fifth  international 
Zoological  Congress  at  Berlin,  and  read  a  long  paper  entitled 
*  Mechanismus  und  Vitalismus '  on  August  16,  1901. l  In  this 
paper  Biitschli  remarks  :  '  The  machine  theory  regards  it  as 
possible,  though  for  the  moment  only  to  a  very  limited  extent, 
to  account  for  the  forms  and  phenomena  of  life  on  the  lines  of 
complex  physico-chemical  conditions.  Vitalism,  on  the  con- 
trary, denies  this  possibility.  The  vitalist  is  convinced  that 
the  physico-chemical  action  of  inorganic  nature  is  not  sufficient 
to  account  for  organic  life,  that  an  altogether  peculiar  action, 
unknown  to  inorganic  nature,  must  exist  in  the  world  of  organic 
life.'  Biitschli  states  the  question  clearly  and  accurately,  but 
unfortunately  we  cannot  say  as  much  for  his  arguments  in 
favour  of  the  machine  theory.  I  listened  to  what  he  said  with 
attention,  and  read  a  report  of  it  afterwards  still  more  atten- 
tively, but  I  discovered  only  one  real  piece  of  evidence  in  favour 
of  the  machine  theory  as  an  explanation  of  life,  and  this  one 
piece  of  evidence  occurred  in  the  closing  words  of  his  dis- 
course :  '  Of  all  the  phenomena  of  life  we  can  understand  only 
what  admits  of  a  physico-chemical  explanation.' 

Professor  Biitschli  will,  I  hope,  forgive  me  for  saying  that 
this  kind  of  evidence  seems  to  me  quite  unintelligible.  If  it 
were  accurate,  the  thoughts  of  the  speaker  would  be  pronounced 
unintelligible  for  himself  as  well  as  for  his  hearers  and  readers. 
According  to  his  own  opinion,  his  thoughts  undoubtedly 
belong  to  the  category  of  phenomena  of  life.  He  ought, 
therefore,  first  to  give  us  a  physico-chemical  explanation  of 
his  own  process  of  thought,  before  he  calls  upon  us  to  under- 
stand his  defence  of  the  machine  theory  ! 

Biitschli  was  certainly  arguing  in  a  circle,  and  thus  his 
arguments  had  no  logical  force.  He  confused  the  ideas  of 
'  to  understand  '  and  '  to  give  a  physico-chemical  explanation,' 
and  regarded  them  as  synonymous,  but  I  must  protest  against 
being  required  to  accept  this.  Either  he  assumed  that  the 
phenomena  of  life,  considered  scientifically,  admitted  only  of 
a  physico-chemical  explanation — which  was  exactly  what  he 

1  See  Verhandlungen,  pp.  212-235. 


undertook  to  prove — or  he  did  not  assume  it,  and  then  he  has 
simply  not  given  us  the  evidence  to  prove  that  the  phenomena 
of  life  have  no  special  vital  laws  governing  them,  over  and 
above  what  is  physical  and  chemical.  It  is  time  for  people 
to  give  up  attempting  to  combat  the  vitalist  theory  with  such 
threadbare  arguments. 

In  the  interests  of  modern  biology  I  must  enter  a  further 
protest  against  Butschli's  entirely  ungrounded  assertion,  that 
we  can  understand  only  what  admits  of  chernico-physical 
explanation,  and  can  understand  it  only  as  far  as  it  can  be 
explained  on  these  lines.  If  this  were  true,  the  scientific 
value  of  the  greatest  biological  triumphs  of  the  present  day 
would  be  absolutely  nothing.  Are  we  in  a  position  to  give  a 
physico-chemical  explanation  of  the  processes  of  indirect 
karyokinesis,  of  fertilisation,  and  of  ontogeny  ?  Are  they 
therefore  simply  unintelligible  to  us  ?  No,  they  are  not ; 
for  we  understand  these  phenomena  chiefly  by  considering 
their  purpose  and  not  their  mechanical  cause.  Just  as  we  can 
understand  why  a  key  of  a  particular  shape  can  turn  in  a  lock, 
without  needing  to  know  by  what  mechanical  process  the  key 
and  the  lock  have  been  made,  so  we  can  grasp  the  significance 
in  fertilisation  and  development  of  the  processes  involved  in 
karyokinesis,  although  we  do  not  know  their  chemico-physical 
causes.  The  assertion  that  the  scientific  intelligibility  _of  a 
biological  process  is  limited  by  the  knowledge  we  possess  of 
its  physico-chemical  causes,  is  therefore  false  and  misleading, 
as  well  as  materialistic.  A  reasonable  explanation  of  biological 
phenomena  cannot  be  given,  unless  they  are  observed  from 
both  the  teleological  and  the  causal,  mechanical  points  of  view, 
since  both  are  worthy  of  equal  consideration.1 

An  opinion  identical  with  my  own  was  expressed  by  L. 
Khumbler  in  an  address  delivered  at  the  seventy-sixth  meeting 
of  German  naturalists  and  physicians  at  Breslau :  '  The 
mechanical  processes  of 'the  cell  do  not  exhaust  the  powers  of 
a  living  cell,  but  concern  it  only  on  its  physico-mechanical 

Other  advocates  of  the  machine  theory  have  not  been 

1  On  this  subject  see  also  J.  Reinke,  Philosophie  der  Botanik,  1905,  chapter 
iii,  *  Kausalitat  und  Finalitat ' ;    also  '  Neovitalismus  und   Finalitat   in  der 
Biologic '  (Biolog.  Zentralblatt,  1904,  Nos.  18  and  19,  pp.  577-601). 

2  Naturwissenschaftliche  Rundschau,  1904,  Nos.  42  and  43,  p.  549. 


much  more  successful  in  adducing  satisfactory  evidence  to 
support  it.  Max  Verworn,  a  famous  physiologist,  writes  as 
follows  in  the  introduction  to  his  '  Zeitschrift  fur  allgemeine 
Physiologie'  (Vol.  I),  when  attacking  neovitalism  and  defending 
the  machine  theory  :  '  The  principles  of  action  must  be  the 
same  everywhere,  as  long  as  we  move  in  a  material  world.' 

But  why  ?  Can  this  be  decided  at  all  a  priori  ?  Must  not 
the  question,  whether  the  principles  underlying  inorganic  and 
organic  action  are  identical  or  not,  be  answered  by  experience  ? 
Experience  tells  us  that  the  vital  processes  are  of  such  a  kind 
as  not  to  admit  of  any  purely  mechanical  explanation.  There- 
fore a  vitalist  is  justified  in  saying  :  '  The  vital  processes  are 
governed  by  laws  of  their  own,  which  are  superior  to  chemico- 
physical  activity.'  By  his  method  of  defending  the  machine 
theory  Verworn  has  really  cut  away  the  ground  from  under 
his  own  feet.  He  asserts  that  purely  mechanical  principles 
must  be  equally  applicable  to  living  and  to  lifeless  bodies, 
and  he  goes  on  to  prove  the  truth  of  this  assertion  by  saying 
that  '  physiology  can  never  be  anything  but  physics  and 
chemistry,  i.e.  the  mechanics  of  the  living  body.'  Therefore 
physiology,  as  a  special  branch  of  biology,  is  quite  superfluous  ; 
we  may  quietly  let  it  drop,  and  incorporate  it  with  physics  and 
chemistry — though  perhaps  Verworn,  being  one  of  our  most 
eminent  physiologists,  will  hardly  agree  to  this. 

If  physiology  were  to  be  nothing  more  than  applied  physics 
and  chemistry  ;  if  the  whole  scientific  value  of  physiology 
were  to  be  measured  by  its  success  in  tracing  all  living  action 
back  to  chemico-physical  causes,  then  indeed  modern  physio- 
logy with  its  imposing  achievements  would  be  in  a  sad  plight. 
G.  von  Bunge  says  in  his  famous  manual  of  human  physiology 
('  Lehrbuch  der  Physiologie  des  Menschen,'  II,  1905,  3) :  '  The 
opponents  of  vitalism  and  adherents  of  the  mechanical  explana- 
tion of  life  are  accustomed  to  justify  their  views  by  maintaining 
that,  the  further  physiology  advances,  the  more  successful 
are  they  in  referring  to  physical  and  chemical  laws  those 
phenomena  which  used  to  be  ascribed  to  some  mystical  vital 
force  ;  it  is  therefore  now  only  a  matter  of  time,  and  eventually 
the  whole  vital  process  will  appear  to  be  a  complicated  set  of 
movements,  governed  solely  by  the  forces  of  inanimate  nature. 
It  seems  to  me,  however,  that  the  history  of  physiology  teaches 


us  the  exact  opposite,  and  I  maintain  that  the  supporters  of 
the  machine  theory  are  wrong.  The  more  thoroughness, 
acumen,  and  impartiality  we  bring  to  bear  upon  our  examination 
of  the  phenomena  of  life,  the  more  do  we  perceive  that 
processes,  for  which  we  had  thought  it  possible  to  account  by 
means  of  physics  and  chemistry,  are  of  jifarmore  complex 
character,  and  for  the  present  defy  every^  attempt  "to  explain 
them  in  a  mechanical  sense.'  Bunge  had  previously  declared 
that  the  machine  theory  of  the  present  day  would  inevitably 
drive  us  towards  the  vitalism  of  the  future,  and  he  was  quite 
right.  Oskar  Hertwig  uses  similar  language  in  his  '  Allgemeine 
Biologie'  (1906),  p.  551,  where  he  says:  '  The  development 
of  the  eye,  the  ear,  and  the  larynx,  as  well  as  of  the  bones,  has 
hitherto  not  been  explained  on  mechanical  lines,  in  fact,  we 
may  say  the  same  of  every  process  of  development ;  for  every- 
where we  meet  with  a  factor  outside  the  scope  of  mechanical 
knowledge,  although  it  is  the  most  important  of  all,  and 
this  factor  is  the  activity  of  the  cell-organism.' 

1  But,'  say  the  champions  of  the  machine  theory,  '  vitalism 
directly  contradicts  the  universally  recognised  law  of  mechani- 
cal  energy.  If  there  were  a  special  vital  activity,  it  would 
violate  trie  law  of  the  conservation  of  a  constant  amount  of 
energy  in  the  universe  —  and  therefore  we  cannot  accept 
the  theory  of  vitalism.'  What  answer  can  we  give  to  this 
argument  ? 

The  law  of  energy  in  its  original  form  is  a  purely  mechanical 
law,  ancfcan  trier eiore_a,ppAy  only  to  the  operation  of  mftpJiarii- 
cal  factors.  It  is  applicable  to  psychical  and  vital  factors 
only  in  so  far  as  they  make  use  of  mechanical  agencies  in  doing 
their  own  work,  and  no  further.  Whoever  has  recourse  to 
the  law  of  energy  in  order  to  prove  a  psychical  or  vital  action 
impossible,  is  either  silently  assuming  that  all  action  in  Jhe 
universe  must  be  essentially  mechanical, — and  then  he  is 
taking  for  granted  what  it  was  his  business  to  prove — or  his 
whole  line  of  proof  is  useless. 

^  The  assumption  of  a  special  vital  action  would  be  really 

contradictory  to  the  law  of  energy  only  if  the  operation  of  the 
vital  principle  either  increased  or  diminished  the  fixed  amount 
ofmechanicalljriergy ;  but  this  is  a  complete  misrepresentation 
of  true  vitalism.  We  need  no  old-fashioned  '  vital  force  ' 


acting  like  a  deus  ex  macJiina,  pushing  and  pulling  and  inter- 
fering with  mechanical  factors,  but  we  require  a  vital  principle, 
which  as  causa  formalis  enables  the  atoms  and  molecules  of  trie 
living  body  to  accomplish  their  chemico-physical  tasks  with  a 
definite  vital  aim.  All  the  mechanical  work  performed  may 
be  put  down  exclusively  to  the  chemico-physical  factors,  and 
not  to  the  vital  principle,  therefore  it  is  impossible  for  the 
latter  to  violate  the  law  of  the  conservation  of  energy. 

The  only  correct  view  of  the  laws  of  life,  which  constitute  the 
essential  difference  between  living  organisms  and  inorganic 
natural  bodies,  was  stated  centuries  ago  by  the  Aristotelian 
philosophers  (see  p.  238),  and  has  recently  been  adopted  by 
eminent  naturalists  of  our  own  day.1  Especial  mention 
must  be  made  of  Hans  Driesch,3  a  great  embryologist,  who 
has  declared  himself  a  supporter  of  the  '  Autonomy  of  the 
Vital  Processes,'  and  has  lately  expressly  described  the  vital 
or  formal  principle,  as  one  corresponding  to  Aristotle's 

J.  Beinke,  the  well-known  botanist,  speaks  of  dominants,  j 
which  are  closely  akin  to  the  idea  of  entelechies.3     These  state-  ! 
ments  may  suffice  to  weaken  the  objections  raised  against 
vitalism  by  the  upholders  of  the  machine  theory,  and,  on  the 
other  hand,  to  give  a  correct  idea  of  what  vitalism  really  is. 

If  we  are  now  asked  the  question  whether  the  assumption 
of  a  special  vital  law,  controlling  the  chemico-physical  agencies, 
is  absolutely  necessary,  in  order  to  supply  a  reasonable  explana- 
tion of  the  embryological  processes  described  in  this  section, 
we  may  answer  shortly  :  *  The  assumption  of  a  vital  principle 
is  absolutely  necessary  in  order  to  account  for  the  phenomena 
of  development.'"" 

I  have  already  alluded  to  the  inadequacy  of  the  attempts 
made  by  J.  Loeb  and  others  to  explain  the  cleavage  process  of 

1  On  this  subject  see  Hans  Malfatti,  '  t)ber  die  Chemie  des  Lebens '  (Die 
Kultur,  1905,  Part  I,  pp.  41-49). 

2  Ergebnisse  der  neueren  Lebens forschung,  14  ;   see  also  by  the  same  author, 
Organische  Regulationen,  Leipzig,  1901,  and  Die  Seek  als  elementarer  Naturfaktor, 
Leipzig,  1903. 

3  Die  Welt  als  Tat,  Berlin,  1903,  pp.  275-292  ;  Einleitung  in  die  theoretische 
Biologie,  Berlin,  1901,  chapters  19  and  20.      'Die  Dominantenlehre '    (Natur 
und  Schule,  1903,  Parts  6  and  7).     See  also  Reinke's  more  recent  work,  'Der 
Neovitalismus  und  die  Finalitat  in  der  Biologie  '  (Biolog.  Zentralblatt,  XXIV, 
1904,   Nos.   18  and   19,  pp.   577-601) ;   also   Philosophie  der  Botanik,    1905, 
chapter  iv. 

B  2 


the  ovum  on  purely  mechanical  lines  (see  p.  222),  I  have 
referred  to  dependent  differentiation  and  to  redifferentiation 
or  readjustment  as  facts  supporting  the  theory  of  epigenesis, 
and  have  shown  in  several  places  (pp.  229,  230,  &c.,  and  235), 
that  we  can  account  for  these  facts  only  if  the  whole  process 
of  development  is  dominated  by  the  idea  of  the  whole  that  is 
to  be  produced — a  form  of  expression  frequently  used  by 
Korschelt  and  Heider  in  their  excellent  *  Lehrbuch  der  vergleich- 
enden  Entwicklungsgeschichte.' 

We  cannot  dispense  with  a  teleological  interpretation  of 
the  processes  of  development ;  they  are  absolutely  incom- 
prehensible, unless  we  assume  the  existence  of  a  formal  principle 
controlling  the  mechanical  agencies,  and  directing  them  to 
the  aim  of  producing  an  organism  capable  of  life. 

But  is  it  altogether  impossible  to  regard  the  fertilised  ovum 
from  the  point  of  view  of  the  preformation  theory,  as  a  wonder- 
fully delicate  and  complicated  machine,  set  in  motion  by 
purely  mechanical  agencies  and  effecting  the  regular  con- 
struction of  the  organism  in  the  process  of  development  ? 
This  machine  theory  of  life  -was  once  upheld  by  Hans  Driesch, 
but  he  has  recently  subjected  it  to  a  very  searching  criticism 
and  condemned  it  as  quite  untenable.  In  his  '  Ergebnisse  der 
neueren  Lebensforschung'  (p.  15),  he  writes  : '  Eggs  are  the  result 
of  an  extremely  complicated  formative  process  ;  therefore 
each  egg  might  be  considered  as  a  very  complex  piece  of 
machinery,  though  so  small  as  to  be  invisible  to  the  naked  eye. 
Now  in  the  course  of  the  ontogeny  of  an  individual,  all  the 
eggs  have  been  formed  from  one  cell,  by  division.  How  can 
a  complex  piece  of  machinery  go  on  dividing  and  yet  remain 
complete  ?  It  is  impossible,  and  therefore,  in  this  department 
also,  the  machine  theory  breaks  down.' 

In  fact  a  machine,  at  once  so  delicate  and  so  ingeniously 
constructed,  able  spontaneously  to  divide  itself  a  hundred 
times,  and  yet  to  preserve  in  all  its  parts  the  power  to  become 
a  complete  machine  again  automatically,  would  be  so  wonderful 
a  piece  of  mechanism  as  to  be  absolutely  inconceivable. 

The  machine  theory  of  life  breaks  down  in  the  equipotential 
systems  (see  p.  227)  no  less  than  in  the  development  of  the 
ovum.  Let  us  refer  to  a  statement  made  on  p.  231  with  regard 
to  the  blastula  of  the  sea-urchin  egg.  Such  a  blastula  may  be 


cut  up  in  any  direction,  and  each  piece  will  grow  into  a  complete 
blastula  ;  in  fact  every  one  of  the  808  cells  forming  the 
blastula  is  capable  of  exchanging  its  original  function  with  any 
other  cell  of  the  same  blastula.  Now  imagine  a  machine  consist- 
ing of  808  parts  ;  hack  the  machine  to  pieces,  and  see  if  each 
single  piece  is  able  '  by  means  of  physico-chemical  factors  '  to 
complete  itself  automatically,  and  produce  a  whole  machine 
able  to  work.  A  machine,  capable  of  doing  this,  is  again 
something  absolutely  inconceivable. 

I  may  quote  from  Driesch  l  another  classical  instance 
showing  that  the  machine  theory  of  life  is  absolutely  untenable. 
He  made  a  series  of  experiments  on  an  Ascidian,  Clavellina 
,  a  rather  highly  organised  creature,  which  he 

describes  as  follows  :  '  Clavellina  is  about  an  inch  long,  and 
its  body  consists  of  three  chief  parts  ;  at  the  top  is  an 
extremely  large,  basket-shaped  branchial  sac,  with  openings 
for  water  to  flow  in  and  out  ;  in  the  middle  is  a  slender  portion 
of  the  body,  which  contains  the  stomodseum  and  proctodaeum, 
and  behind  it  we  see  the  intestinal  sac,  containing  the  stomach, 
intestine,  heart,  organs  of  propagation,  &c. 

'  If-  a  Clavellina  is  cut  in  two,  across  the  narrow  part  of  its 
body,  so  that  the  branchial  and  the  intestinal  sacs  are  separated, 
each  of  these  two  parts  is  able  in  three  or  four  days  to  grow 
into  a  complete  animal,  as,  by  means  of  regeneration  from  the 
wounded  surface,  the  branchial  sac  supplies  itself  with  an 
intestinal  sac,  and  the  intestinal  sac  with  a  branchial  sac. 
But  the  branchial  sacs  of  Clavellina  do  not,  when  isolated, 
always  behave  in  the  way  just  described.  About  half  of  them, 
and  especially  those  belonging  to  small  specimens,  arrive  at 
the  formation  of  a  new  whole,  but  by  a  totally  different  method. 
They  do  not  begin  by  producing  any  new  formation  at  all,  but 
they  undergo  a  complete  transformation.  The  organisation 
of  the  branchial  sac,  its  ciliated  stigmata,  apertures,  &c., 
gradually  vanish,  and  after  five  or  six  days  it  is  no  longer  pos- 
sible to  trace  any  organisation  at  all,  the  creatures  look  like 
uniform  white  balls  ;  in  fact,  when  I  first  saw  these  shapeless 

1  *  Studien  iiber  das  Kegulationsvermogen  der  Organismen  '  :  6.  '  Die 
Restitutionen  der  Clavellina  lepadiformis  '  (Archiv  /.  Entwicklungsmechanik, 
XIV,  1902,  Parts  1  and  2,  pp.  247-287)  ;  see  also  Ergebnisse  der  neueren  Lebens- 
forschung,  pp.  10-12. 


masses  before  me,  I  thought  they  were  dying,  if  not  actually 
dead.  But  such  is  not  the  case.  They  may  remain  for  as 
long  as  two  or  three  weeks  in  this  shapeless  condition  ;  then, 
one  day,  they  begin  to  show  signs  of  life  and  to  stretch,  and 
in  two  or  three  more  days  they  are  again  complete  Ascidians, 
with  branchial  sac,  intestinal  sac,  &c.  They  are  absolutely 
new  creatures,  having  no  part  in  common  with  the  original, 
but  made  of  the  same  material.  Their  branchial  sacs  are  not 
the  old  ones  that  were  cut  off,  but  are  much  smaller,  with 
fewer  channels,  and  fewer  and  smaller  apertures. 

'  The  organisation  of  the  isolated  branchial  sac  seems  to 
have  been  reduced  to  undifferentiated  material,  out  of  which, 
as  in  embryonic  development,  a  complete  little  organism  has 
been  formed.  Sections  made  by  the  microtome  through  the 
balls  undergoing  retrogressive  transformation  show  that  the 
change  of  differentiated  into  undifferentiated  substance  had 
gone  very  far.  We  now  come  to  the  most  important  point 
in  the  results  of  our  experiments  on  isolated  branchial  sacs 
of  Clavellina.  Not  only  is  the  isolated  branchial  sac  itself  able 
to  become  a  little  Ascidian  by  means  of  retrogressive  trans- 
formation and  regeneration,  but  it  may  be  cut  in  half  in  any 
direction,  so  as  to  form  an  upper  and  a  lower,  or  a  front  and 
a  back  half,  and  each  half  still  possesses  the  power  to  undergo 
retrogressive  transformation,  and  to  develop  into  a  little 
Ascidian,  complete  in  every  detail  of  its  organisation.  This 
is  undoubtedly  an  extremely  strange  phenomenon  in  organic 

So  far  I  have  quoted  from  Driesch.  Let  us  now  compare 
the  capacity  of  reformation  possessed  by  the  branchial  sacs  or 
portions  of  them,  undergoing  retrogressive  transformation, 
with  the  favourite  example  of  a  machine  of  very  complex 
structure,  such  as  the  upholders  of  the  machine  theory  regard 
as  essentially  equivalent  to  a  living  organism.  Let  us  imagine 
that  we  break  the  machine  in  pieces,  and  choose  one  piece, 
which  we  break  again,  for  closer  observation.  After  a  few  days 
this  piece  falls  into  a  confused  mass  of  fragments,  so  that 
nothing  of  the  original  parts  of  the  machine  can  be  recognised. 
It  remains  in  this  condition  for  some  weeks,  and  then  suddenly 
begins  to  move,  the  various  bits  of  iron  come  together  quite 
spontaneously  and  form,  not  the  original  piece  of  the  machine 


which  gave  rise  to  the  mass  of  fragments,  but  a  new  and 
complete  little  machine,  constructed  on  the  same  lines  as  the  old 
one.  Any  one  would  say  that  nothing  short  of  witchcraft  could 
accomplish  this,  and  it  is  a  fact  that  a  Clavellina,  acting  in 
accordance  with  the  machine  theory  of  life,  would  never 
naturally  succeed  in  performing  such  a  feat.  We  declare, 
therefore,  that  the  machine  theory,  which,  in  spite  of  the 
accomplishment  of  such  wonders,  persists  in  regarding  the 
Clavellina  as  a  mere  machine,  makes  large  demands  upon  our 
credulity.  But  as  we  are  convinced  that  natural  causes,  and 
not  magic  arts,  underlie  the  marvels  of  development,  we  come 
to  this  conclusion  :  Vitalism  is  the  only  philosophical  theory 
of  life  that  is  in  accordance  with  reason,  for  it  d.oes  not  regard 
the  livingorganism  as  a  mere  machine,  but  it  knows  howjfcp 
find  the  architect  residing  in  it ! 

1  In  the  smallest  cell  we  have  all  the  problems  of  life  before 
us.'  These  words  of  Bunge's  l  have  found  abundant  confirma- 
tion in  the  preceding  pages.  A  diminutive  egg-cell,  once 
fertilised,  contains  already  the  design  of  the  whole  complex 
organism  which  is  to  proceed  from  it,  and  it  contains  it  in  a 
way  that  defies  all  purely  mechanical  explanationT  The  study 
of  ontogeny  has  brought  us  to  the  same  conclusions  as  those 
which  we  expressed  at  the  end  of  Chapter  VI  (pp.  177,  &c.), 
although  by  another  road,  that,  namely,  of  modern  embryology. 
In  Chapter  VI,  the  results  of  microscopical  study  of  the 
phenomena  of  fertilisation  and  heredity  led  us  to  assume 
the  existence  of  internal  laws  of  development,  controlling  the 
maturation-divisions  of  the  germ-cells  and  their  union  in 
the  course  of  fertilisation,  and  directing  these  processes  to 
a  definite  end.  We  found  that  the  chromosomes  should 
probably  be  regarded  as  the  chief  material  bearers  of  heredity, 
but  their  morphological  function  was  by  no  means  a  satis- 
factory explanation  of  the  real  problem  of  development.  Even 
if  the  supporters  of  the  chromosome  theory  really  succeeded, 
by  means  of  most  accurate  microscopical  observations,  in 
showing  conclusively  that  their  theory  agreed  with  the  results 
of  embryological  physiology ;  even  if  they  were  able  to 
express  the  amazing  processes  of  regeneration  in  Clavellina  by 

1  Lehrbuch  der'.Physiologie  des  Menschen,  II,  11. 


a  complicated  formula  of  chromosomes  (which  would  have 
to  surpass  in  ingenuity  the  System  of  the  Universe,  the  out- 
come of  Laplace's  giant  intellect) — they  would  still  not  have 
solved  the  mystery  of  life,  as  it  is  presented  to  us  by  the  problem 
of  ontogeny.  The  external  aspect  of  the  problem,  and  no 
other,  can  be  dealt  with  by  means  of  microscopical  observation, 
and  by  considering  the  morphological  peculiarities  of  chromo- 
somes of  definite  shape,  dividing  in  definite  ways,  and  distri- 
buting themselves  in  definite  numbers  to  the  various  cells 
of  the  new  organism — we  have  still  not  touched  the  other  side 
of  these  embryological  processes,  which  is  concerned  with 
their  interior  dynamics.  The  physiological  part  played  in 
the  maturation  and  fertilisation  of  the  germ-cells,  and  in  the 
subsequent  cleavage-divisions  of  the  embryo,  by  the  'chromo- 
somes, as  bearers  of  heredity,  upon  one  another  and  upon  the 
cell -plasm,  goes  far  beyond  the  scope  of  the  most  subtle  machine 
theory,  and  reaches  far  into  the  domain  of  the  mysterious 
conformity  to  vital  laws  that  manifests  itself  in  living  creatures. 
In  studying  the  processes  both  of  fertilisation  and  of  develop- 
ment, we  must  necessarily  assume  the  existence  of  some  inner 
causes  working  harmoniously  to  one  common  end,  and  thus 
only  shall  we  understand  the  physiological  importance  of  the 
chromosomes.  If,  on  the  one  hand,  these  material  parts, 
visible  only  under  the  microscope,  are  really  the  smallest 
wheels,  setting  the  wonderful  clockwork  of  life  in  action  from 
generation  to  generation,  and  if  the  movements  of  these  wheels 
are  due  immediately  to  some  still  unknown  chemico-physical 
laws  acting  upon  the  molecules  of  albumen  and  nuclein  in 
the  cells,  we  must  remember  that,  on  the  other  hand,  they 
are  living  wheels,  and  it  is  only  from  their  uniform  action, 
which  has  the  whole  vital  process  as  its  aim,  that  the  chromo- 
some theory  of  the  future  will  ever  be  able  to  supply  a  really 
satisfactory  explanation  of  the  phenomena  of  life.  This 
uniform  action,  however,  must  have  a  uniform  interior  cause, 
and  this  we  perceive  in  the  vital  principle  of  the  organism 
to  which  I  have  already  alluded. 

In  Chapter  VII  we  considered  a  number  of  facts,  that  led 
us  to  accept  this  immanent  teleological  principle,  whilst  they 
revealed  the  impossibility  of  spontaneous  generation.  Now 
that  we  have  surveyed  the  results  of  modern  embryology,  the 


acceptance  of  this  same  principle  has  been  shown  to  be  necessary 
in  a  far  higher  degree. 

The  vital^rincjple,  that  controls  what  goes  on  in  a  diminu- 
tive fertilised  ovum,  is  at  the  same  time  the  architect,  directing 
the  course  of  the  whole  resulting  process  of  development,  and 
bringing  it  to  completion  by  means  of  the  mechanical  agencies 
that  are  subordinate  to  him.  But  this  little  architect  is  not 
himself  an  intelligent  being  ;  he  has  power  to  act  in  the  various 
cells  and  in  the  whole  organism,  and  to  direct  all  to  their  aim, 
but  he  does  so  in  virtue  of  the  laws  which  a  higher  intelligence, 
superior  to  our  universe,  imposed  upon  living  matter  when  the 
first  organisms  came  into  being.  This  higher  intelligence  we 
call  a  personal  Creator.  The  necessity  for  assuming  the 
existence  of  this  first  cause  for  all  conformity  to  law  in  organic 
life — would  remain  undiminish^HT  if  f.he  machine'  theorists 
succeeded  in  accountiri^  for  all  the  vital  process^  without,  a. 
vitai  ' principle.  Only  an  architect  of  infinite  intelligence 
could  possibly  construct  a  machine  capable  of  developing, 
growing,  and  propagating  itself  for  millions  of  years  by  means 
of  purely  mechanical  agencies.  The  reasons  for  regarding  the 
machine  theory  of  life  as  untenable  are  therefore  not  theological, 
but  scientific.  Unicellular  living  creatures  and  the  fertilised 
ovum  and  the  organism  proceeding  from  it,  all  have  in  them- 
selves the  vital  principles,  which  uniformly  direct  the  action 
of  the  chemico-physical  forces  of  the  single  atoms  towards  the 
higher  aim  of  life. 

Our  praise  is  due,  not  to  these  diminutive,  unconscious 
architects,  but  to  the  eternal   creative    Spirit  that  has  con- . 
nected  them  with  matter. 




Its  hypothetical  character  (p.  253).  Evidence  in  favour  of  race-evolu- 
tion (p.  254).  Positive  scientific  evidence  is  all  in  favour  of 
polyphyletic  evolution  (p.  255). 


Fourfold  use  of  the  name  (p.  257).  What  view  must  we  take  of  Dar- 
winism ?  Darwin's  theory  of  selection  is  not  the  whole  of  the  doctrine 
of  evolution  (p.  259).  Haeckel's  testimony  to  this  fact  (p.  261). 
Nee-Darwinism  and  Neo-Lamarckism  (p.  263).  The  Darwinian 
cosmogony  (Haeckelism)  is  wrong  (p.  265).  Equally  wrong  is  its 
application  to  man  (p.  266). 


It  is  not  concerned  with  the  origin  of  life  (p.  268).  Its  task  is  to  investi- 
gate the  facts  and  causes  connected  with  the  different  series  of 
organic  forms  (p.  270). 



Kant  and  Laplace's  theories  regarding  the  development  of  the  celestial 
bodies.  The  geological  formation  of  our  earth  and  its  natural 
causes  (p.  273).  The  sequence  of  species  of  plants  and  animals 
in  the  course  of  the  history  of  our  earth  is  to  be  explained  by 
natural  causes,  i.e.  by  evolution,  not  by  repeated  acts  of  creation 
(p.  275).  Instances  from  palaeontology  (p.  276). 


First :  Philosophical  limitations  (p.  279).  Recognition  of  a  personal 
Creator.  His  action  regarding  the  origin  of  primitive  organisms, 
their  number  and  mode  of  evolution  being  unknown  to  us  (p.  280). 
A  creative  act  is  indispensable  to  account  for  the  mind  of  man  (p.  283). 
Second:  Scientific  limitations  (p.  285).  Hypothesis  and  theory. 
Theories  of  permanence  and  descent  (p.  285).  When  did  the 
first  organisms  come  into  being  ?  (p.  288).  Monophyletic  or 
polyphyletic  evolution  ?  (p.  291).  The  causes  of  race -evolution 
(p.  294).  Problems  still  to  be  solved  relating  to  the  course  and 
causes  of  race  evolution  (p.  295). 


The  natural  species  is  a  series  of  forms  of  systematic  species  genetically 
connected  (p.  296).  Scientific  and  philosophical  importance  of  the 
distinction  between  natural  and  systematic  species  (p.  297).  The 
theory  of  evolution  is  perfectly  compatible  with  the  dogma  of  creation 
(p.  299). 


1  An  article  published  in  the  Biologisches  Zentralblatt  for  1891  (Nos.  22,  23), 
dealing  with  the  evolution  of  the  varieties  of  Dinarda,  gave  rise  to  a  number 
of  unfair  remarks  upon  my  attitude  towards  the  theory  of  evolution.  I  thought 
it  possible  to  show  that  the  varieties  of  the  Dinarda  beetle,  living  among  our 




THE  ontogeny  of  organisms,  which  we  discussed  in  the 
previous  chapter,  is  a  direct  object  of  scientific  observation. 
That  the  seed  of  a  rose  develops  into  a  rosebush,  and  a  hen's 

ants,  were  not  strictly  speaking  species  at  all,  but  races,  standing  on  various 
levels  with  regard  to  the  formation  of  species.  Further,  I  was  able  to  show  that 
the  differences  in  our  various  kinds  of  Dinarda  appeared  to  be  characteristics  due 
to  adaptation  of  their  way  of  life  to  that  of  the  various  kinds  of  ants  who  were 
their  hosts.  In  this  article  I  mentioned  shortly  several  other  facts,  that  I 
had  observed  in  the  course  of  my  special  study  of  the  inquilines  among  ants 
and  termites,  and  that  I  considered  were  arguments  in  favour  of  a  modified 
theory  of  evolution.  I  remarked  emphatically  that  I  regarded  the  theory  as 
justified  only  in  so  far  as  it  is  really  based  on  ascertained  facts  in  the  case  of 
definite  series  of  forms  ;  I  altogether  refused  to  accept  the  so-called  '  Postu- 
lates,' which  the  monists  set  up  in  the  name  of  the  theory  of  evolution. 

In  spite  of  this  important  reservation,  a  reviewer  in  the  Schlesische  Zeitung 
of  January  21,  1902,  ventured  to  claim  me  simply  as  a  supporter  of  the  theory 
of  descent.  In  the  Supplement  to  the  Allgemeine  Zeitung  for  June  17,  1902 
(No.  136),  a  longer  article  appeared  by  Dr.  K.  Escherich,  entitled,  'A  Jesuit  as 
an  adherent  of  the  theory  of  descent.'  It  is  true  that  my  own  opinions  were 
reproduced  in  it  with  praiseworthy  accuracy,  and  that  attention  was  drawn 
explicitly  to  my  not  regarding  as  justifiable  the  extension  of  the  theory  of 
evolution  to  man.  But  the  reviewer  went  on  to  express  a  hope  that  the 
theory  would  soon  be  accepted  without  reservation  by  me  and  the  whole 
Catholic  Church  !  I  think,  therefore,  that  I  am  absolutely  bound  in  this 
place  to  state  clearly  what  I  am  ready  to  accept  in  the  theory  of  evolution, 
and  what  I  reject  as  mere  additions  from  Darwinian  and  monistic  sources. 
Moreover,  in  his  review  Dr.  Escherich  spoke  of  me  as  an  opponent  of  the  other 
advocates  of  the  Christian  cosmogony,  and  especially  of  all  other  Catholic 
theologians,  and  this  is  certainly  not  the  truth.  It  is  not  a  dogma  that  every 
species  owes  its  existence  to  a  particular  act  of  creation.  More  than  twenty- 
five  years  ago  Father  Knabenbauer.  S.J..  contributed  a  very  careful  article 
on  *  Glaube  und  Deszendenztheorie '  ('  Faith  and  the  Theory  of  Descent') 
to  Stimmen  aus  Maria-Loach  (XIII,  1877).  On  p.  72  of  this  article  he  says  : 
*  Faith  does  not  forbid  us  to  assume  Trial  the  now  existing  varieties  of  plants 
and  animals  are  derived  from  some  few  original  forms.'  Professor  Schanz 
expresses  similar  views  in  his  Apologie  des  Christentums,  1895,  to  which  attention 
was  drawn  by  articles  in  the  supplement  to  the  Germania,  July  3,  1902, 
No.  150,  and  the  Deutsche  Eeichszeitung,  No.  326.  More  than  twenty  years  ago, 
the  Stimmen  aus  Maria-Loach  several  times  contained  emphatic  warnings  to 
be  careful  to  distinguish  Darwinism  and  the  theory  of  evolution;  although 
the  former  must  be  rejected,  there  are  many  facts  to  support  the  theory  that 
organic  species  have  developed  within  definite  series  of  forms. 

Extracts  from  Escherich's  review  concerning  my  attitude  towards  the 
theory  of  descent  were  subsequently  reprinted  in  the  Frankfurter  Zeitung  of 
July  18,  1902,  No.  197  ;  in  the  Deutsche  Zeitung,  No.  168 ;  and  in  the  Bohemia 
of  July  20,  No.  198 ;  with  the  unfortunate  title  '  Ein  Jesuit  als  Anhanger  des 
Darwinismus  '  ('  A  Jesuit  as  an  adherent  of  Darwinism ').  In  order  to  remove 
all  misunderstandings  that  may  have  arisen  in  consequence  of  these  newspaper 
reports,  I  intend  to  make  a  clear  and  detailed  statement  here  of  my  opinions 
on  the  subject  of  evolution,  which  have  also  been  expressed  in  a  number 
of  lectures  of  a  popular  scientific  nature,  delivered  in  various  German  towns 
and  in  Luxemburg  since  the  year  1901.  It  was  easy  to  foresee  that  the  extreme 
Darwinists  would  attack  my  views,  but  I  can  notice  only  those  attacks  which 
have  some  foundation  on  facts.  Further  remarks  on  this  subject  will  be 
found  at  the  beginning  of  this  book  in  the  '  Few  Words  to  my  Critics/  and 
at  the  end,  in  the  appendix  containing  my  Innsbruck  lectures. 


egg  into  a  chicken,  are  facts  of  everyday  occurrence.  Therefore 
the  study  of  individual  ontogeny,  which  concerns  itself  with  the 
way  in  which  the  various  living  organisms  of  the  present  day 
come  into  being,  is  in  its  nature  an  empirical  science.  In  it 
hypotheses  and  theories  begin  only  at  the  point  where  we 
seek  a  deeper  insight  into  the  laws  and  causes  of  the  actual 
development  which  we  can  observe. 

But  with  the  race  history  of  organisms  it  is  otherwise. 
The  science  dealing  with  this  subject  is  generally  called  simply 
the  doctrine  of  evolution  or  the  theory  of  descent.  It  is  not 
empirical,  but  by  its  very  nature  it  is  a  hypothesis,  which 
has  grown  into  a  theory  by  the  aid  of  the  circumstantial 
evidence  adduced  in  its  support.  I  propose  to  do  my  best  to 
give  my  readers  a  clear  idea  of  what  it  implies. 

Hoses  and  poultry  have  not  always  existed,  both  in  fact  are 
of  very  recent  date ;  the  earliest  representatives  of  the  family 
to  which  our  poultry  belong  are  found  in  the  upper  Eocene,  i.e. 
in  the  Tertiary  period  of  the  earth's  history.  Whence  came  the 
first  rose,  and  the  first  hen  ?  Were  they  suddenly  created,  just 
as  we  know  them,  or  were  they  developed  from  other  kinds  of 
plants  and  animals  that  lived  before  them  ?  If  so,  how  was 
this  development  or  evolution  effected  ?  These  questions  are 
very  simple  and  obvious,  and  yet  they  are  of  great  importance 
in  our  comprehension  of  the  vegetable  and  animal  world 
about  us.  The  Flora  which  now  covers  the  face  of  the  earth 
with  leaves  and  blossoms,  and  the  Fauna  which  now  under 
various  forms  inhabits  sea  and  land,  are  not  the  original  occu- 
pants of  our  world,  but  late-born  epigoni.  They  took  the  place 
of  other  plants  and  animals  which  lived  in  the  same  world 
before  them,  and  are  to  some  extent  known  to  us  through  their 
fossil  remains  ;  and  these  earlier  plants  and  animals  had  other 
predecessors  in  still  more  remote  periods,  and  so  we  may  go  on, 
until  at  last  we  come  to  the  first  and  oldest  forms  of  animal 
and  vegetable  life  on  our  planet.  And  here  again  the  same 
question  confronts  us :  '  Did  the  later  representatives  of  the 
Flora  and  Fauna  come  into  existence  quite  independently  of 
the  earlier  ones,  or  are  they  chiefly  their  modified  descendants  ? ' 

We  know  that  geology  divides  our  earth  into  a  series  of 
strata,  formed  successively  one  after  the  other,  and  arranged 
one  above  the  other. 


I.  Azoic  or  archaic  strata,  containing  no  organic  remains. 
II.  Palaeozoic    strata,    containing    the    earliest    traces    of 
organic  life — 

1.  Cambrian  (including  Pre-Cambrian).  • 

2.  Silurian. 

3.  Devonian. 

4.  Carboniferous  (Coal). 

5.  Permian  (Dyas). 

III.  Mesozoic  strata  (the  middle  ages  of  organic  life) — 

1.  Triassic  (red  sandstone,  shell  lime,  marl). 

2.  Jurassic  (black,  brown,  and  white  Jura  or  Lias  ; 

Middle  Jurassic  or  Dogger  ;    Upper  Jurassic  or 

3.  Cretaceous  (Chalk). 

IV.  Caenozoic  strata  (the  modem  period  of  organic  life) — 

1.  Tertiary  age  (Eocene,  Oligocene, Miocene,  Pliocene). 

2.  Quaternary  age  (Pleistocene  or  Diluvium,  Present 

or  Alluvium). 

Man,  the  highest  of  all  created  beings,  appeared  only  in  the 
Pleistocene  period  ;  but  the  history  of  animal  and  vegetable 
life  upon  earth  began  thousands,  perhaps  millions  of  years 
before  man's  appearance.  No  human  eye  beheld  the  beginning 
of  the  drama  of  life  on  our  planet,  no  human  eye  watched  the 
thousands  of  scenes  enacted  from  the  moment  when  the  great 
drama  opened,  to  the  moment  when  man  came  forth  as  the  last 
and  noblest  figure  on  the  stage  of  life.  And  now  he  ventures 
boldly  to  look  back  into  the  past  and  survey  the  whole  history 
of  the  evolution  of  organic  life  on  earth.  He  tries  to  find  out 
in  what  order  the  various  forms  of  animals  and  plants  have 
succeeded  one  another,  from  the  earliest  times  down  to  the 
present  day,  and  he  attempts  to  account  for  this  succession 
by  tracing  the  later  forms  back  to  the  earlier,  by  means  of 
natural  evolution  of  species,  genera,  families,  &c. 

It  is  therefore  quite  intelligible  that  this  theory  of  evolution, 
having  as  its  subject  the  conjectural  race-history  of  the  organic 
world,  cannot  be  an  empirical  science,  but  bears,  and  must 
inevitably  bear,  a  hypothetical  character.  But  as  the  human 
spirit  of  research  makes  use  of  facts  as  a  starting  point  for  its 
comparisons  and  deductions,  the  theory  of  evolution  rightly 
claims  to  be  called  a  science,  scientia  rerum  ex  causis  ;  for 


race-evolution,  if  we  accept  it,  enables  us  to  give  a  comparatively 
simple  and  natural  explanation  of  a  number  of  phenomena 
actually  occurring  in  various  departments  of  biology.  Inas- 
much as  it  is  in  a  position  to  offer  the  most  probable  account 
of  these  facts,  we  must  undoubtedly  regard  the  theory  of 
evolution  as  scientific,  although  the  evidence  which  the  scientist 
can  use  in  support  of  the  theory  is  almost  exclusively  circum- 
stantial ;  and  indeed  we  cannot  expect  it  to  be  otherwise, 
for  we  are  dealing  with  the  previous  history  of  the  living 
organisms  known  to  us,  with  a  primaeval  period,  of  which  at  the 
present  day  we  find  only  faint  traces  and  fragmentary  remains. 
Like  a  skilful  advocate,  the  man  of  science  must  carefully 
collect  his  circumstantial  evidence,  and  fit  it  together,  so  as  to 
reconstruct  from  it  a  course  of  events  which  no  one  actually 

The  circumstantial  evidence  in  support  of  race-evolution 
is  of  many  different  kinds.  It  consists  firstly  of  the  facts  of 
palaeontology,  which  offers  us  the  fossil  remains  of  extinct 
animals  and  plants  as  silent  witnesses  to  the  primaeval  history 
of  our  present  Fauna  and  Flora.  We  have  also  the  facts  of 
variation  and  mutation,  which  show  us  how  the  properties 
of  still  existing  creatures  can  be  modified,  and  new  species 
formed.  Comparative  bionomics  shows  us  how  animals  and 
plants  undergo  adaptation  to  one  another,  and  are  influenced 
by  very  various  external  factors,  and  these  facts  enable  us 
to  infer  how  the  altered  relations  have  come  about.  The 
facts  of  comparative  morphology  also,  the  points  of  likeness 
in  interior  and  exterior  structure  that  exist  among  members 
of  definite  families,  these  too  are  quite  explicable  if  we  may 
assume  that  they  have  a  common  descent.  Lastly,  there  are 
the  facts  concerned  in  the  ontogeny  of  the  individual,  which 
incidentally  reveals  to  us  traces  of  former  race-evolution. 
In  short,  the  various  branches  of  zoology  and  botany — both 
empirical  sciences — supply  innumerable  pieces  of  circum- 
stantial evidence,  of  which  the  theory  of  descent  makes  use. 
If  it  does  so  in  a  critical  and  careful  manner,  we  have  a  scientific 
foundation  for  the  theory  of  evolution,  although  we  have  no 
wish  to  deny  its  hypothetical  character.  If,  however,  the 
circumstantial  evidence  is  used  in  a  superficial  and  fanciful 
way,  and  involves  groundless  generalisations  and  reckless 


jumping  at  conclusions,  we  have,  instead  of  a  scientific  theory  of 
evolution,  merely  a  fantastic  semblance  of  it,  which  is  pre- 
tentious enough  to  put  forward  its  arbitrary  statements  as 
historical  truths. 

The  very  subject-matter  of  the  theory  of  evolution  shows — 
and  I  am  careful  to  emphasise  it  again — that  it  is  indeed  based 
upon  many  results  of  the  empirical  sciences,  but  can  never 
be  itself  an  empirical  science,  and  will  always  remain  a  hypo- 
thetical explanation  of  observed  facts,  and  as  such  it  has  risen 
to  the  rank  of  a  theory.  We  must,  however,  always  be  careful 
to  distinguish  hypotheses  and  facts  ;  and  this  is  especially 
necessary,  because  the  theory  of  evolution  in  many  respects 
stretches  beyond  the  domain  of  natural  science  into  that 
of  natural  philosophy,  and  it  is  often  difficult  to  define  the 
boundaries  of  each.  For  this  reason  we  must  act  cautiously 
with  regard  to  the  *  postulates '  which  so-called  monism 
has  set  up  in  the  name  of  the  theory  of  evolution,  for 
they  are  not  based  on  scientific  facts,  but  on  materialistic 

Without  entering  upon  a  full  account  of  the  history  of  the 
theory  of  evolution,  I  may  shortly  sketch  the  outlines  of  the 
problem  with  which  we  are  going  to  deal. 

In  order  to  explain  the  origin  of  the  existing  species  of 
plants  and  animals,  we  have  to  assume  one  of  two  things.  We  / 
may  assume  that  the  systematic  species  (e.g.  lion,  tiger,  polar 
bear)  are  invariable — apart  from  the  formation  of  varieties 
and  breeds  within  the  species — and  that  they  were  created 
originally  in  their  present  form.  Or  we  may  assume  that  2. 
the  systematic  species  are  variable,  and  constitute  definite 
lines  of  descent,  within  which  an  evolution  of  species  has  taken 
place  during  the  geological  periods.  The  first  of  these  assump- 
tions belongs  to  the  theory  of  permanence,  the  second  to  the 
theory  of  evolution  or  descent.  In  the  latter  we  must  make 
a  further  distinction  between  monophyletic  and  polyphyletic 
evolution.  According  to  the  monophyletic  theory,  all  organ- 
isms  have  originated  in  one  single  primitive  cell,  or  perhaps  there 
is  one  pedigree  for  all  animals  and  one  for  all  plants,  each 
having  one  primitive  ancestor.  According  to  the  polyphyletic 
theory  there  are  several  pedigrees  for  both  plants  and  animals, 
independent  of  one  another,  but  each  one  going  back  to  one 


special  primitive  form  as  its  starting  point.1  In  the  following 
pages  we  shall  see  that  the  latter  assumption  alone  can  claim 
to  have  any  positive  scientific  probability — and  we  shall  see, 
moreover,  that  this  assumption  is  perfectly  reconcilable  with 
the  Christian  doctrine  of  the  Creation. 


For  over  forty  years  a  conflict  has  been  raging  in  the  in- 
tellectual world,  which  both  sides  have  maintained  with  great 
vehemence  and  energy.  The  war-cry  on  one  side  is  *  Evolution 
of  Species,'  on  the  other  '  Permanence  of  Species.'  No  one 
could  fail  to  be  reminded  of  that  other  great  intellectual 
warfare  regarding  the  Ptolemaic  and  the  Copernican  systems, 
which  began  about  three  hundred  and  fifty  years  ago,  and 
raged  with  varying  success  for  over  a  century,  until  finally 
the  latter  prevailed.  Perhaps  the  present  conflict  between  the 
theories  of  evolution  and  permanence  only  marks  a  fresh  stage 
in  that  great  strife,  and,  if  so,  how  will  it  finally  be  decided  ? 

The  contest  that  we  have  to  consider  was  stirred  up  by 
Charles  Darwin,  when  he  published  his  book  on  the  '  Origin 
of  Species '  about  the  middle  of  last  century.  The  theories 
advanced  by  Lamarck  and  Geoffroy  St.  Hilaire  at  the  end 
of  the  eighteenth  and  the  beginning  of  the  nineteenth  centuries 
may  be  regarded  as  causing  preliminary  skirmishes,  but 
Cuvier's  powerful  attacks  soon  succeeded  in  overthrowing  the 
new  ideas  of  evolution  (see  p.  28).  It  was  not  until  the 
year  1859 2  that  the  great  battle  began,  which  has  received 
its  name  from  the  commander-in-chief  of  the  attacking  army, 
Charles  Darwin.  The  warfare  with  which  we  are  now  con- 
cerned centres  round  Darwinism,  so-called. 

I  say,  so-called  Darwinism.  A  few  words  of  explanation 
are  absolutely  necessary.  The  thick  smoke  of  the  powder, 
which  hid  the  battlefield  from  our  gaze,  is  gradually  dispersing, 

1  It  is  of  secondary  importance  to  consider  how  many  individuals  there 
were  of  each  primitive  form.     The  chief  point  is  that  the  Anlage  for  evolution 
in  each  primitive  form  differed  from  those  of  the  primitive  forms  of  other 
lines  of  descent. 

2  The  first  English  edition  of  Origin  of  Species  was  published  in  November 
1859,  as  Darwin  himself  stated,  although  1858  is  sometimes  erroneously  given 
as  the  date  of  its  publication.     See  Francis  Darwin,  Life  and  Letters  of  Charles 
Darwin,  I  (London,  1888),  p.  84. 


and  it  is  much  easier  now  than  it  was  twenty  or  thirty  years 
ago  to  survey  the  armies  on  both  sides  and  to  judge  of  their 
positions,  their  strength  and  their  mode  of  fighting,  and  to 
value  rightly  what  they  have  achieved  and 'what  they  still 
have  to  accomplish.  It  now  appears  that  the  number  of 
scientific  combatants  gathered  under  Darwin's  banner  is 
still  comparatively  small.  By  far  the  greater  number  of 
supporters  of  what  was  once  called  Darwinism  are  now  ranged 
under  the  standard  of  the  theory  of  evolution,  and  no  longer 
under  that  of  Darwinism.  These  troops  form  the  rank  and 
file,  but  Ernst  Haeckel  is  the  leader  of  a  corps  of  free-lances 
and  freebooters,  conspicuous  for  the  disturbance  that  they 
cause  in  the  name  of '  Science.'  l 

Their  weapons  are  not,  however,  of  the  best  and  noblest 
sort,  and  their  aim  is  not  the  triumph  of  truth,  but  rather  the 
plunder  of  the  Christian  camp,  that  they  suspect  to  be  situated 
somewhere  in  the  rear  of  their  opponents'  position.  But  victory 
does  not  incline  to  them  ;  with  their  wooden  swords  they 
bring  upon  themselves  one  defeat  after  another,  and  only 
succeed  in  hindering  the  triumph  of  the  picked  troops  of  really 
scientific  men,  who  fight  with  better  weapons  on  the  side  of  the 
theory  of  evolution. 

It  is  time,  however,  to  explain  in  simple  words  the  simile 
of  the  battle  which  has  presented  itself  to  our  sight. 

If  we  want  to  answer  the  question  :  'What  are  WQ  to  think 
about  Darwinism  ?  '  we  must  first  of  all  try  to  grasp  clearly 
the  different  senses  in  which  this  name  is  used. 

The  first  and  most  obvious  way  in  which  the  word  Darwinism  f 
is  used,  is  to  designate  the  theory  of  selection,  put  forward  by 
Charles  Darwin  ;  i.e.  the  special  form  of  the  theory  of  descent, 
which  traces  back  the  evolution  of  organic  species  to  natural 
selection,  as  its  chief,  if  not  its  only  cause.  Man  uses  his 
intelligence  to  produce  artificial  breeds  of  domestic  animals, 
by  selecting  for  breeding  those  that  show  the  peculiarities 
that  answer  his  purpose.  Darwin,  however,  assumes  the 
occurrence  of  a  natural  selection  with  no  purpose  at  all ;  he 
thinks  that,  by  its  means,  in  the  struggle  for  existence  some 
varieties  prove  better  able  to  hold  their  own  than  others,  and 

1  On  January  11,  1906,  they  founded  the  '  German  Monistic  League  ' 
(Deutscher  Monistenbund)  in  Jena,  under  Haeckel's  presidency. 


their  peculiarities  are  accentuated  by  transmission  to  following 
generations,  whereas  the  varieties  that  are  less  capable  of 
self-preservation  die  out.  This  is  the  fundamental  idea  of 
Darwin's  theory  of  selection. 

The  word  Darwinism  received  a  second  meaning  when  it 
was  applied  to  an  extension  of  the  theory  of  selection  to  a  new 
and,  as  it  was  called,  philosophical  theory  of  the  universe.  It 
was  assumed  that  not  only  the  organic  species,  but  the  whole 
orderly  arrangement  of  the  world,  had  arisen  out  of  an  originally 
lawless  chaos  by  means  of  accidental  '  Survival  of  the  Fittest/ 
In  Germany  Ernst  Haeckel  has  been  the  chief  founder  and 
champion  of  this  Darwinian  theory  of  the  universe,  and  there- 
fore it  is  also  known  as  Haeckelism.  It  bears  the  misleading 
name  of  '  Realistic  Monism,'  but  it  would  be  better  designated 
'  Materialistic  Atheism.' 

The  third  use  of  the  word  Darwinism  proceeded  from  the 
extension  to  man  of  Darwin's  theory  of  selection.  In  this 
sense,  the  theory  that  man  is  descended  from  beasts  is  called 
Darwinism,  whether  it  be  Vogt's  theory  of  the  descent  of  man 
from  apes,  or  some  more  modern  opinion  of  the  same  kind. 
According  to  this  '  Darwinian '  view  of  man,  he  is  in  both 
body  and  soul  nothing  but  a  beast,  that  has  accidentally 
reached  a  higher  point  of  development  than  his  fellows.  The 
first  to  deduce  this  conclusion  from  the  Darwinian  System 
was  an  Englishman,  Huxley,  in  his  work  '  Evidence  as  to 
Man's  Place  in  Nature '  (London,  1863).  He  was  followed  by 
Haeckel  in  his  '  Natiirliche  Schopfungsgeschichte '  (1868). 
It  was  not  until  1871  that  Darwin  himself  made  up  his 
mind  to  extend  his  theory  to  man  in  his  '  Descent  of  Man.' 
This  book  is  really  the  weakest  of  all  Darwin's  scientific  works. 

In  1887  Wiedersheim  attempted  to  give  a  detailed  anatomi- 
cal foundation  for  the  descent  of  man  from  apes  in  his  book 
on  the  structure  of  man  as  evidence  of  his  past  ('  Der  Bau 
des  Menschen  als  Zeugnis  fur  seine  Vergangenheit,'  3rd  ed., 
Tubingen,  1902).  An  excellent  refutation  of  this  piece  of 
fiction  was  given  in  1892  by  0.  Hamann  in  an  article  on  'Darwin- 
ism and  the  Theory  of  Evolution '  ('  Darwinismus  und  Entwick- 
lungslehre')  (see  p.  108,  &c.).  The  weakness  of  the  Darwinian 
methods  of  proof  is  thoroughly  displayed  by  J.  Eanke  in  his 
work  on  Man  ('  Der  Mensch/  2  vols.). 


The  fourth  and  last  meaning  attached  to  the  name  Darwin- 
ism is  due  to  its  having  been  applied  first  to  a  particular 
form  of  the  theory  of  descent,  and  afterwards  transferred  to 
the  theory  of  descent  in  general.  Although  this  use  depends 
upon  a  confusion  of  ideas,  the  name  is  still  in  popular  language 
applied  to  the  whole  doctrine  of  the  evolution  of  organic 
species,  as  opposed  to  the  theory  of  permanence,  which  assumes 
that  the  systematic  species  never  change,  and  were  created 
originally  in  their  present  form.  In  this  sense,  therefore,  every 
student  of  nature,  who  declares  the  species  in  any  one  genus 
of  animals  or  plants  to  be  related  to  one  another,  is  a  Darwinist, 
though  erroneously  so-called. 

This  last  application  of  the  name  Darwinism  ought  to  be 
given  up,  as  it  only  leads  to  confusion.  It  is  based — and  I 
must  again  emphasise  the  fact — upon  a  logical  blunder,  for  it 
confuses  the  theory  of  evolution  as  a  whole  with  a  particular 
form  of  it.  This  blunder  was  pardonable  forty  years  ago,  when 
Darwin's  theory  of  evolution  was  the  only  one  known,  but  it  is 
pardonable  no  longer.  At  the  present  day  it  is  unfair  to 
identify  the  ideas  conveyed  by  the  names  *  Darwinism  '  and 

*  Theory  of  Evolution,'  and  it  is  done  only  with  a  special 
intention  ;  the  adherents  of  Darwinism,  on  the  one  hand,  have 
recourse  to  this  device  in  order  to  propagate  their  obsolete 
theory  in  popular  circles,  and  the  opponents  of  the  theory  of 
evolution,  on  the  other  hand,  try  to  annihilate  every  attempt 
to  question  the  permanence  of  species,  by  hurling  at  it  the 
epithet  *  Darwinism.' 

It  will  now  be  an  easier  task  for  us  to  answer  the  question  : 

*  What  are  we  to  think  about  Darwinism  ?  '     We  see  that  the 
question  resolves  itself  into  four. 

1.  What  are  we  to  think  of  Darwin's  Theory  of  Selection  ? 

2.  What  are  we  to  think  of  the  extension  of  Darwin's 

Theory  of  Selection,  so  as  to  make  of  it  a  realistic 
and  monistic  theory  of  life  ? 

3.  What  are  we  to  think  of  the  application  to  man  of 

Darwin's  Theory  of  Selection  ? 

4.  What  are  we  to  think  of  the  Theory  of  Evolution  as 

opposed  to  that  of  Permanence  ? 

It  is  the  object  of  our  present  discussion  to  supply  an 
answer  to  the  last  of  these  questions,  and  I  can  deal  with 

s  2 



the  first  three  only  briefly,  for  they  have  often  been  answered 
before,  and  admit  also  of  much  shorter  answers  than  the  fourth. 
First. — Modern  science  can  hardly  be  said  to  take  into 
account  Darwin's  theory  of  selection  as  the  exclusive  form 
of  the  theory  of  evolution.  It  is  full  of  weak  spots,  to  which 
attention  was  drawn  as  early  as  1874  by  Albert  Wigand,1 
and  it  is  impossible  any  longer  to  avoid  recognising  them. 
In  the  first  place  the  theory  of  selection  is  in  principle  not 
satisfactory,  for  natural  selection  may  be  able  to  .destroy 
what  is  inexpedient,  but  not  to  produce  what  is  expedient. 
Therefore  it  simply  leaves  to  chance  the  origin  of  advantageous 
modifications,  which  lead  to  the  formation  of  new  species.  A 
theory  based  on  chance  is  worthless  as  affording  an  explanation 

y  ^  oi  conformity  to  law  in  nature.  In  the  second  place,  most  of 
t^ie__variatiQns  which  serve  as  the  groundwork  of  classifica/ETon 
are  biologically  indifferent,  and  do  not  affect  the  individual 
or  the  species  in  the  struggle  for  existence  ;  they  can  therefore 
not  be  due  to  natural  selection  in  their  breeding,  because  they 

(7  ^  present  no  points  d'appui  on  which  it  can  work.  In  the  third 
place,  in  order  to  account  for  the  formation  of  one  new  species, 
this  theory  requires  innumerable,  almost  imperceptible  varia- 
tions to  have  existed  for  immense  periods  of  time  and  to  have 
been  gradually  accumulating  and  intensifying.  This  con- 
tradicts known  facts  of  palaeontology,  for  the  Fauna  and  Flora 
of  remote  ages  display  a  definite  system  of  classes,  orders, 
families,  genera  and  species,  just  as  do  those  of  the  present  day, 
and  not  a  chaos  of  imperceptibly  slight  variations,  such  as 
the  theory  of  selection  requires. 

For  these  reasons  most  naturalists  have  by  this  time 
abandoned  the  theory  in  its  exclusive  form.  An  eminent 

1  Der  Darwinismus  und  die  Naturforschung  Newtons  und  Cuviers,  I.  Cf. 
also  G.  Wolff,  '  Beitrage  zur  Kritik  der  Darwinschen  Lehre  '  (Biolog.  Zentral- 
blatt,  X,  1891,  Nos.  15  and  16) ;  0.  Hamann,  Entwicklungslehre  und  Darwinis- 
mus, Jena,  1892,  chapter  ix  ;  A.  Goette,  '  t)ber  den  heutigen  Stand  des  Dar- 
winismus '  (Die  Umschau,  1898,  Part  5) ;  Aug.  Pauly,  Wahres  und  Falsches 
an  Darwins  Lehre,  Munich,  1902  ;  Lamarckismus  und  Darwinismus,  Munich, 
1905  ;  Max  Kassowitz,  *  Die  Krisis  des  Darwinismus  '  (Die  Zukunft,  February 
15,  1902) ;  E.  Dennert,  Am  Sterbelager  des  Darwinismus,  Stuttgart,  1905  and 
1906;  H.  Kranichfeld,  'Die  Wahrscheinlichkeit  der  Erhaltung  und  der 
Kontinuitat  giinstiger  Varianten  in  der  kritschen  Periode  '  (Biolog.  Zentral- 
Uatt,  1905,  No.  20  ;  1906,  No.  8) ;  Chr.  Schroder,  '  Kritische  Beitrage  zu  den 
strittigen  biologischen  Fragen  der  Gegenwart '  (Natur  und  Schule,  V,  1906, 
Part  6,  pp.  233-247) ;  0.  Zacharias,  '  Planktonforschung  und  Darwinismus  ' 
(Zoolog.  Anzeiger,  XXX,  1906,  Nos.  11,  12,  pp.  381-388). 


modern  zoologist,  Dr.  Hans  Driesch^  condemned  it  perhaps 
rather  harshly  in  the  BiologiscJies  Zentralblatt  for  1896,  p.  355, 
when,  in  speaking  of  Darwinism,  he  said  :  '  It  is  a  matter  of 
history,  like  that  other  curiosity  of  our  century,  Hegel's 
philosophy.  Both  are  variations  on  the  theme  "  how  to 
take  in  a  whole  generation,"  and  neither  is  very  likely  to 
give  ages  to  come  a  high  opinion  of  the  latter  part  of  our 
century.'  In  the  same  publication  for  1902,  p.  182,  he  says : 
'  For  men  of  clear  intellect,  Darwinism  has  long  been  dead, 
and  the  last  argument  brought  forward  in  support  of  it  *  is 
scarcely  more  than  a  funeral  oration  in  accordance  with  the 
principle  De  mortuis  nil  nisi  bonum,  and  with  an  underlying 
conviction  of  the  real  weakness  of  the  subject  chosen  for  defence.' 

Professor  Oskar  Hertwig,  Director  of  the  Anatomical  and 
Biological  Institute  at  the  University  of  Berlin,  expressed 
himself  almost  as  strongly  in  an  address  delivered  at  the 
meeting  of  German  naturalists  at  Aix-la-Chapelle,  on  September 
17,  1900,  on  the  growth  of  biological  knowledge  in  the 
nineteenth  century.  He  points  out  the  necessity  of  distin- 
guishing clearly  between  the  theory  of  evolution  and  the 
theory  of  selection,  and  then  continues  (p.  15)  :  *  They 
stand  on  a  very  different  foundation  and  basis,  for  we  might 
say  with  Huxley  :  "  The  theory  of  evolution  would  stand 
where  it  did,  even  if  Darwin's  hypothesis  were  blown  away." 
In  the  former  we  have  a  permanent  achievement  of  our  century, 
based  upon  facts,  and  certainly  worthy  to  be  numbered  among 
the  chief  attainments  of  our  age.'  We  shall  have  to  examine 
later  on  to  what  extent  the  theory  of  evolution  is  really  based 
upon  facts. 

In  one  of  his  lectures  given  in  April  1905,  at  the  Berlin 
Singakademie,  even  Ernst  Haeckel  frankly  acknowledged, 
in  at  least  one  passage,3  that  the  theory  of  natural  selection 
alone  ought  to  be  termed  Darwinism  in  the  stricter  sense,  and 
he  added  :  '  We  cannot  now  discuss  the  extent  to  which  this 
theory  is  justified,  nor  how  far  it  has  been  amended  by  other 

1  The  reference  is  to  a  paper  by  L.  Plate  in  the  VerJiandlungen  der  Deutschen 
Zoologischen  Gesellschajt  for  1899  :  «  Die  Bedeutung  und  Tragweite  des  Darwin- 
schen  Selektionsprinzips.'     The  paper  has  since  appeared  in  an  enlarged  form 
with    title :    Uber    die    Bedeutung    des    Darwinschen   Selektionsprinzips   und 
Probleme  der  Aribildung,  Leipzig,  1903. 

2  Der  Kampf  urn,  den  Entwicklungsgedanlcen,  Berlin,  1905,  p.  20. 


newer  theories,  such  as  Weismann's  Germ-plasm  theory  (1884) 
and  de  Vries'  theory  of  mutation.'  He  did  not  refer  to  this 
delicate  question  in  his  later  lectures.  The  passage  is  particu- 
larly noteworthy,  because  Haeckel,  as  the  *  Prophet  of  Dar- 
winism,' has  for  nearly  forty  years  been  confusing  Darwinism 
and  the  theory  of  evolution  to  suit  his  own  ends,  and  has 
extolled  Darwin's  theory  of  selection  as  the  highest  intellectual 
achievement  of  the  nineteenth  century,  because  it  teaches 
us  how  to  understand  design  in  nature  without  recognising 
a  wise  Creator  !  And,  after  all,  Haeckel  himself  finally  acknow- 
ledges that  the  confusion  between  Darwinism  and  the  theory 
of  evolution  is  a  mistake,  and  he  can  scarcely  find  any  scientific 
justification  for  the  theory  of  selection.  I  feel  inclined  to 
put  on  Darwin's  lips  the  words  *  Et  tu,  Brute,'  uttered  by  the 
dying  Caesar  ! 

This  confession  on  Haeckel's  part  must  have  been  very 
unwelcome  to  those  who  support  Darwinism  from  the  point  of 
view  of  popular  science,  and  who  try  to  mislead  the  general 
public  by  confusing  it  with  the  theory  of  evolution.  One  of 
them,  E.  H.  France,  in  a  work  entitled  '  Die  Weiterentwicklung 
des  Darwinismus '  (*  The  further  development  of  Darwinism '), 
1904,1  has  tried  to  represent  all  the  progress  made  by  the 
theory  of  evolution  since  Darwin's  time,  and  even  modern 
vitalism  itself,  as  a  triumphant  '  further  development '  of 
Darwinism,  whereas  in  reality  he  is  uttering  a  sort  of  funeral 
oration  over  it. 

That  Darwinism  and  the  theory  of  evolution  are  two 
essentially  different  things  is  quite  evident  from  the  evolution 
theories  of  Mivart,2  Wigand,3  Kolliker,4  Heer,5  Nageli,6  Eimer,7 

1  Gemeinverstdndliche  Darwinistische  Vortrdge  und  Abhandlungen,  published 
by  W.  Breitenbach,  Part  12.  To  show  the  method  of  proof  adopted  by 
France,  I  may  mention  that  in  the  above-mentioned  work  (p.  24),  by  means 
of  unmistakable  falsification  of  a  quotation  from  Stimmen  aus  Maria-Laach, 
he  tries  to  make  out  that  the  Jesuit  Father  Wasmann  is  a  supporter  of  the 
theory  of  permanence,  in  order  thus  to  render  '  Jesuitical  science  '  harmless 
from  his  point  of  view. 

a  The  Genesis  of  Species,  London,  1871. 

s  Die  Genealogie  der  Urzellen  als  Losung  des  Deszendenzproblems,  Brunswick, 

4  '  Allgemeine  Betrachtungen  zur  Deszendenzlehre  '  (Abhandl.  der  Senken- 
bergschen  Naturforschenden  Gesellsch.,  VIII,  1872,  pp.  206-237). 

5  Urwelt  der  Schweiz,  Zurich,  1883,  chapter  18. 

6  Mechanisch-physiologische  Abstammungslehre,  Leipzig,  1884. 

7  Die  Entstehung  der  Arten,  I,  Jena,  1888  ;  II,  Leipzig,  1897, 

NEO-DAftWINlSM  268 

de  Vries,1  Gulick  3  and  others,  who  either  attack  Darwin's 
principle  of  selection,  or  impose  very  strict  limitations  upon  it.3 
Kolliker  and  Eimer's  theories  unfortunately  resemble  Dar- 
winism in  having  a  mechanical  and  monistic  basis,4  but  they 
have  the  great  merit  of  combating  it  on  scientific  grounds,  for 
they  admit  internal  causes  of  evolution  as  the  chief  factors 
in  the  hypothetical  phylogeny  of  living  organisms.  Eimer's 
researches  into  evolution  proceeding  towards  some  definite 
aim  (orthogenesis)  were  continued  after  his  death  by  his 
pupils,  Countess  Maria  von  Linden  and  Dr.  Fickert.  It  is 
worth  noticing  that  E.  Strasburger,  the  well-known  botanist, 
who  formerly  upheld  the  theory  of  selection,  has  recently 
given  it  up  very  decidedly.5  It  is  true  that  there  are  still 
at  the  present  day  in  Germany  some  eminent  zoologists, 
especially  Professor  August  Weismann  at  Freiburg  im  Breisgau, 
who  profess  to  defend  Darwin's  theory  of  the  all-importance 
of  natural  selection,6  but  on  closer  examination  Weismann's 
'  Neo-Darwinism '  also  appears  to  be  gradually  beating  a 
retreat,  the  first  stage  in  which  is  marked  by  W.  Koux's  '  His- 
tonal  Selection,'  or  selection  of  the  tissues  ;  Eoux  tries  to 
supply  the  deficiencies  of  the  principle  of  selection  by  trans- 
ferring Darwin's  personal  selection  to  the  struggle  among 
the  various  parts  in  the  living  organism.  When,  therefore,  in 
1895,  Weismann  propounded  his  theory  of  germinal  selection, 
as  the  last  bulwark  of  the  principle  of  selection,  he  acknowledged 
that  not  Darwin's  natural  selection,  but  interior  causes  of 
eVolutioii,  iiilisJ^J^  t.hq  nhiflf  fflp-tor  in  a.n  orderly  evolution 
of  the  organic  world.7 

1  Die  Mutationstheorie,  Versuche  und  Beobachtungen  uber  die  Entstehung  von 
Arten  im  Pflanzenreich,  I,  Brunswick,  1901  ;   II,  ibid.,  1903. 

2  Rev.    John    T.    Gulick,    Evolution   racial   and    habitudinal    (Theory  of 
Divergence),  Washington,  Carnegie  Institution,  1905. 

3  In  his  Konvergenz  der  Organismen,  Berlin,  1904,  H.  Friedmann  has  even 
attempted  to  substitute  the  principle  of  divergence  for  that  of  descent.     I 
cannot  say  that  I  think  his  attempt  successful ;    the  two  principles  are  com- 
plementary to  one  another,  but  neither  can  take  the  place  of  the  other. 

4  With  regard  to  Kolliker's  theory  see  an  article  by  Professor  Stolzle,  '  A.  von 
Kollikers  Stellung  zur  Deszendenzlehre,'   Miinster  i.   W.,  1901   (Natur  und 
Offenbarung,  1901).     On  the  principles  underlying  Eimer's  theory  of  ortho- 
genesis see  Wasmann,  '  Die  Entstehung  der  Arten  nach  Eimer '  (Natur  und 
Offenbarung,  1889,  pp.  44,  &c.). 

5  Cf.  Jahrbucher  fur  wissenschajtliche  Botanik,  1902,  pp.  518,  &c. 

6  Cf.  Weismann's    'Lectures  on  the  Evolution    Theory,'    Eng.     trans., 
London,  1904. 

7  See  remarks  in  Chapter  VI,  p.  176. 


In  the  scientific  theory  of  descent,  selection  is  now  regarded 
as  a  subordinate  factor  of  more  or  less  importance,  but  it 
cannot  take  the  place  of  the  interior  factors  determining 
the  evolution  of  the  race,  in  fact  it  presupposes  their  existence. 
0.  Hertwig  remarks  very  aptly  on  this  subject  ('  Allgemeine 
Biologie,'  1906,  p.  620) :  *  It  seems  to  me  perfectly  plain  that 
no  advantage  is  gained  by  the  use  of  such  phrases  as  "  Struggle 
between  the  parts  of  an  organism,"  "  intraselection,"  "  histo- 
logical  selection,"  "  germinal  selection,"  they  do  not  enable  us 
better  to  understand  the  processes  of  organic  nature.  They 
teach  us  no  more  about  what  goes  on  within  the  organism 
than  a  chemist  would  learn  about  the  formation  of  any  organic 
compound,  if  he  were  to  content  himself  with  using  such  a 
phrase  as  "  the  struggle  of  the  molecules  in  a  test-tube  "  for 
explaining  some  chemical  process.' 

Neo-Lamarckism  stands  in  direct  contrast  to  Weismann's 
Neo-Darwinism.  In  1809,  Jean  Lamarck  wrote  his  '  Philosophie 
Zoologique,'  in  which  he  traced  the  development  of  species  to 
direct  functional  adaptation,  viz.  to  the  principle  of  the 
use  or  disuse  of  organs  ;  from  this  followed  inevitably  the 
theory  that  the  qualities  thus  acquired  by  the  individual 
could  be  transmitted  to  his  descendants.  Charles  Darwin 
did  not  by  any  means  exclude  the  principle  of  direct  adaptation 
and  the  power  of  transmitting  acquired  qualities,  but  he 
assigned  to  them  less  importance  than  to  natural  selection. 
Weismann,  however,  and  the  Neo-Darwinists  after  him, 
denied  the  possibility  of  direct  adaptation  and  the  trans- 
mission of  acquired  qualities.  According  to  them,  nothing 
was  inherited  but  modifications  working  directly  upon  the 
germ-plasm.  This  view  was  opposed  by  the  Neo-Damarckians 
under  the  guidance  of  Herbert  Spencer  and  K.  von  Nageli, 
who  upheld  the  principle  of  direct  adaptation,  and  maintained 
that  acquired  qualities  could  be  transmitted.  Among  the 
modern  representatives  of  Neo-Lamarckism  we  may  mention 
particularly  two  zoologists,  viz.  Oskar  Hertwig ]  and  L. 
Hatschek,2  E.  Koken,  a  palaeontologist,3  and  B.  von  Wettstein, 

1  Allgemeine  Biologie,  Jena,  1906,  esp.  chapters  27-30. 

2  '  Hypothese  der  organischen  Vererbung '  :   an  address  delivered  at  the 
seventy-seventh  meeting  of  German  naturalists  at  Meran,  Leipzig,  1905. 

3  '  Palaontologie  und  Deszendenzlehre  '   ( Verhandl.    der  73  Versammlung 
deutscher  Naturforscher  zu  Hamburg,  I,  Leipzig,  1902,  pp.  221,  &c.). 


a  botanist.1  As  a  matter  of  fact,  both  direct  adaptation  and 
selection  seem  to  take  part  in  the  processes  of  evolution ;  the 
former  to  a  greater  degree  than  the  latter,  because  it  results 
from  the  interior  laws  of  evolution,  whilst  selection  only  plays 
the  negative  part  of  eliminating  the  unfit.  It  is  self-evident 
that  only  those  modifications  can  be  hereditary  which  in 
some  way  have  stamped  themselves  on  the  germ-plasm,  but 
how  and  to  what  extent  the  characteristics  acquired  by 
individuals  are  transmitted  to  the  germ-plasm,  is  a  very  dark, 
mysterious  question.2  Oskar  Hertwig  in  his  'Allgemeine 
Biologie,'  p.  598,  has  made  a  suggestion  which  is  certainly 
very  important  in  connexion  with  the  theory  of  evolution. 
He  says  :  '  Is  it  not  possible  that,  just  in  the  same  way  as  the 
multicellular  organism  develops,  by  epigenesis  from  the  egg, 
so,  when  we  survey  the  matter  from  the  point  of  view  of  the 
theory  of  descent,  each  species  may  develop  in  accord- 
ance with  a  permanent,  regular  principle  of  progress,  not 
as  the  plaything  of  chance,  but  with  the  same  interior  neces- 
sity as,  in  ontogeny,  the  blastula  must  grow  out  of  the 
gastrula  ?  ' 

Second. — We  can  give  a  still  shorter  answer  to  the  question 
regarding  the  extension  of  Darwin's  theory  of  selection,  so  as 
to  make  of  it  a  realistic  and  monistic  cosmogony3 — it  is  simply 
a  mischievous  act  committed  in  the  name  of  science. 

It  is  mischievous  philosophically,  because  it  traces  back 
the  origin  of  all  conformity  to  law  in  the  natural  order  to  a 
denial  of  all  conformity  to  law  as  to  its  primary  cause.  It 
is  mischievous  theologically,  although  it  vaunts  itself  to  be 

1  Uber  direJcte  Anpassung,  Vienna,  1902  ;  Der  Neolamarckismus  und  seine 
Beziehungen  zum  Darwinismus,  Jena,  1903. 

2  In  his  book  *  Lamarckismus  und  Darwinismus,  Munich,  1 905,  A.  Pauly 
aims  at  adducing  fresh  psychological  evidence  in  support  of  Lamarckism. 
His  ideas  on  teleology  are,  however,  mostly  wrong  and  psychologically  without 

3  The  physical  arguments  in  favour  of  this  extension  are  stated  in  Haeckel's 
Riddle,  of  the  Universe,  but  they  have  been  submitted  to  a  very  destructive 
criticism  in  a  work  entitled  Hegel,  Haeckel,  Kossuth  and  the  Twelfth  Com- 
mandment, by  0.  D.  Chwolson,  Professor  of  Physics  at  the  University  of  St. 
Petersburg,  and  author  of   a   valuable  textbook  of  Physics,  that  has  been 
translated"  into  German.     We  may  assume  that  everyone  knows  the  sharp 
criticisms  pronounced  upon  Haeckel's  Riddle    of    the   Universe  by  Professor 
Paulsen  in  his  Philosophia  militans,  by  Professor  Loofs  in  his  Antihaeckel,  by 
Professor  Seeberg  and  others.     E.  Dennert's  popular  works,  Die   Wahrheit 
uber  Ernst  Haeckel  und  seine  Weltrdtsel  (Halle  a.  8.,  1904)  and  Haeckels  Weltan- 
schauung, Stuttgart,  1906,  are  very  well  worth  reading. 



the  '  Religion  of  the  Future,'  for  it  alters  the  conception  of 
God,  the  most  perfect  Being,  and  reduces  it  to  absolutely 
nothing,  whilst  ostensibly  preserving  it ;  hence  it  would  be 
more  honest  to  call  it  atheism  than  monism.  Finally  Haeckel's 
cosmogony  is  mischievous  socially,  and  constitutes  one  of  the 
greatest  dangers  for  human  society,  inasmuch  as  it  proclaims 
the  *  struggle  for  existence  '  and  the  accidental  '  survival  of 
the  fittest '  to  be  the  only  laws  in  the  natural  order,  and  it 
exalts  them  to  be  the  only  laws  governing  human  society 
also.  Haeckelism  is,  therefore,  the  support  of  anarchy  and  of 
social  democracy,  as  Bebel  once  informed^  us  in  the  German 

Third. — We  saw  that  the  third  use  of  the  name  Darwinism 
was  to  designate  the  application  to  man  of  Darwin's  theory 
of  selection.3  If  man  is  really  nothing  more  than  a  higher 
animal,  if  God  does  not  exist  for  him,  nor  an  immortal  soul, 
nor  any  retribution  beyond  the  grave,  then  human  society 
is  indeed  delivered  over  to  anarchy,  and  the  anarchists  are  the 
only  sensible  people.  But  to  uphold  such  a  doctrine  in  the 
name  of  science  is  worse  than  humbug,  it  is  a  grievous  offence 
against  the  highest  possessions  of  mankind.3  Those  periodicals 
are  guilty  of  participation  in  this  offence,  which  profess  to 
present  science  in  a  popular  form,  and  recklessly  represent 
the  application  of  Darwinism  to  man  as  justified  by  assured 
scientific  results.  Even  men  like  Rudolf  Virchow,  who  do  not 

1  In  his  well-known  speech  on  September  16,  1876,  in  which  he  proved  the 
connexion  between  social  democracy  and  Darwinism,   that  Haeckel  denied, 
Bebel's  words  were  :     '  Gentlemen,   in  my  opinion   Professor  Haeckel,   the 
decided  advocate  of  the  Darwinian  theory,  because  he  does  not  understand 
social  science,  has  no  idea  at  all  that  Darwinism  must  necessarily  promote 
socialism,  and  vice  versa,  socialism  must  harmonise  with  Darwinism,  if  its 
aims  are  to  be  correct.'      Cf.  also  a  little  pamphlet,  Darwinismus  und  Sozial- 
demokratie,  oder  Haeckel  und  der   Umsturz,  Berlin,  1895.     It  is  a  matter  of 
especial  psychological  interest  that  recently  even  anarchists  have  attacked 
the  theory  of  the  struggle  for  existence.     The  Russian  anarchist,  Prince  Peter 
Kropotkin,  has  done  this  in  his  book  on  mutual  help  in  development,  which 
G.  Landauer  translated  into  German,  Gegenseitige  Hilfe  in  der  Entwicldung, 
Leipzig,  1904.     Even  to  men  of  this  type  the  theory  of  selection  is  beginning 
to  seem  untenable,  but  apparently  they  do  not  see  that,  by  acknowledging 
this  fact,  they  are  undermining  the  foundations  of  their  own  social  theories. 

2  A  further  discussion  of  this  subject  will  be  found  in  Chapter  XI. 

3  For  a  scientific  criticism  of  Darwin's  theory  of  the  descent  of  man,  see 
the  works  of  Hamann  and  Ranke,  mentioned  on  p.  258  ;    also  J.  Bumiiller, 
Mensch  oder  Affe  ?  Ravensburg,  1900  ;  C.  Gutberlet,  Der  Mensch,  sein  Ursprung 
und  seine  Entwicklung,  Paderborn,  1903  ;  Wilh.  Schneider,  Gottliche  Weltordnung 
und  religionslose  Sittlichkeit,  Paderborn,  1906. 


claim  to  speak  from  the  point  of  view  of  Christianity,  have 
felt  bound  to  protest  vehemently  against  this  mischievous 


It  is  high  time  for  us  to  go  on  to  the  real  question  under 
discussion,  and  ask :  '  What  are  we  to  think  of  the  theory 
of  evolution  in  itself  ?  Have  the  systematic  species  always 
existed  in  their  present  forms,  or  are  they  mostly  related  with 
other  species,  some  still  existing,  and  others  extinct,  and 
known  to  us  only  by  fossil  remains  dating  from  earlier  ages 
of  the  world  ?  Are  they  the  result  of  an  historical  evolution 
of  the  organic  world,  or  were  they  originally  created  in  their 
present  condition  ?  ' 

In  order  to  be  able  to  deal  with  this  important  question 
objectively  and  impartially,  it  is  indispensable  for  us  to 
disregard  altogether  the  misuse  made  of  the  theory  of  evolution 
by  those  who  distort  it  to  answer  the  purposes  of  atheistic 
materialism.  It  is  much  to  be  regretted  that  this  misuse  of  it 
occurs.  It  is  embodied  in  Haeckelism,  which  is  by  no  means 
a  feather  in  the  cap  of  modern  science.  Nothing  has  more 
injured  the  reputation  of  the  theory  of  descent — as  the  doctrine 
ot  evolution  is  called  in  scientific  circles — than  the  fact  that 
one_  section  of  atheists  and  materialists  have  used  it  as  a 
battering-ram  against  Christianity  ;  nothing  has  done  more  to 
vulgarise  it  and  disfigure  its"  scientific  character  than  this 
misuse  of  it,  which  has  rendered  it  almost  unrecognisable. 
It  is  chiefly  owing  to  this  misuse,  that  those  who  profess  to 
be  Christians  regard  the  theory  of  descent  with  so  much 
suspicion,  and  think  themselves  bound  to  hold  aloof  from  it, 
because  they  confuse  the  anti- Christian  character  thus  given 
it  with  the  essence  of  the  theory  of  evolution.  We  must 
resolutely  put  aside  all  thoughts  of  this  misapplication,  and 
consider  the  doctrine  of  evolution  as  what  it  really  is,  viz, 
a  scientific  theory,  which  we  may  either  accept  or  reject Ton 
its  own  merits., 

nrepeat7"we  have  to  consider  the  doctrine  of  evolution 
as  a  scientific  theory,  which  arises  out  of  the  facts  of  the 


organic  world,  and  seeks  to  offer  the  best  and  simplest  natural 
explanation  of  them,  in  accordance  with  strictly  logical  methods 
of  thought.  We  are  not  concerned  with  that  pseudo-theory  of 
descent,1  which,  starting  from  the  a  priori,  considerations  of  a 
false  philosophy,  takes  as  its  fundamental  axiom :  '  We 
refuse  to  admit  the  existence  of  a  personal  Creator,  and  there- 
fore, whatever  exists,  must  have  developed  itself  by  purely 
mechanical  means.'  No  less  false  than  this  fundamental 
principle  are,  of  course,  the  various  so-called  postulates,  which 
the  pseudo-theory  of  descent  is  fond  of  stating  in  the  name 
of  science.  In  the  name  of  true  science  we  are  forced  to  oppose 
an  emphatic  veto  to  these  postulates,  for  the  methods  of  this 
theory  of  descent  are  utterly  antagonistic  to  those  of  true 
scientific  procedure.  We  must  take  up,  however,  another 
attitude  with  regard  to  the  question  what  we  are  to  think 
of  the  theory  of  evolution,  from  the  point  of  view  of  natural 
science.  We  need  not  feel  any  scruple  about  attempting 
to  answer  this  question,  for  we  lay  down  no  false  postulates 
of  materialism,  but  we  approach  it  taking  as  our  starting 
points  real  facts,  viz.  the  works  of  God  in  nature. 

Why  should  we  fear  to  look  the  truth  in  the  face  ?  We 
know  with  absolute  certainty  that  one  truth  can  never  contra- 
dict another,  therefore  the  recognition  of  what  is  really  true 
in  the  theory  of  evolution  can  tend  only  to  the  glory  of  Him 
who  is  the  highest  and  eternal  Truth.3  Let  us,  therefore,  try 
to  give  an  honest  and  careful  answer  to  the  question  :  '  What 
is  the  scientific  value  of  the  modern  theory  of  evolution  ? 
What  does  it  explain  ?  How  far  is  it  necessary  to  a  scientific 
comprehension  of  the  organic  world  about  us  ?  ' 

Is  the  theory  of  descent  able  to  account  for  the  origin 
of  organic  creatures  and  of  organic  life  on  our  earth  ?  No, 

1  The  advocates  of  Haeckelism  are  doing  their  best  to  identify  this  pseudo- 
theory  of  descent  with  the  scientific  theory  of  evolution.     An  instance  of  this 
was  given  by  H.  E.  Ziegler,  in  an  address  delivered  at  the  seventy-third 
meeting  of  German  naturalists  at  Hamburg  on  September  26,   1901,  and 
printed  at  Jena,  1902,  with  the  title  :    fjber  den  derzeitigen  Stand  der  Deszen- 
denzlehre  in  der  Zoologie.  ^  It  is  the  counterpart  of  Haeckel's  address  delivered 
in  Cambridge  in  1898  :    Vber  unsere  gegenwdrtige  Kenntnis  vom  Ursprunge  des 
Menschen,  Bonn,  1899.     Haeckel's  influence  on  Ziegler  is  plainly  apparent  in 
the   latter's  Hamburg  lecture  (cf.  for  instance  pp.  18,  19,  24,  28,  43,  &c.). 
I  think  it  unnecessary  for  this  reason  to  criticise  Ziegler's  views  mere  fully. 

2  On  this  subject  see  J.  Knabenbauer,  S.  J.,  '  Glaube  und  Deszendenztheorie  ' 
(Stimmen  aus  Maria-Laach,  XIII,  1877,  pp.  71,  &c.). 


it  cannot,  for  it  is  a  theory  of  natural  science,  and  natural 
science  can  tell  us  nothing  of  the  source  of  life  on  our  planet. 
It  only  knows  the  facts  and  the  laws  to  be  deduced  -from  them. 
But,  however  carefully  we  compare  these  laws  with  one 
another,  and  however  skilfully  we  combine  them,  they  give 
us  no  suggestion  of  spontaneous  generation,  i.e.  of  the  spon- 
taneous development  of  living  creatures  from  lifeless  matter  ; 
on  the  contrary,  modern  biology  is  directly  opposed  to  the 
theory  of  spontaneous  generation  (cf .  Chapter  VII,  '  The  Cell 
and  Spontaneous  Generation ').  If,  therefore,  a  modern 
scientist,  acting  not  as  an  investigator  of  nature,  but  as  a 
monistic  *  philosopher,'  appeals  to  natural  science  for  evidence 
that  the  assumption  of  spontaneous  generation  is  *  a  postulate 
of  science,'  he  is  entangling  himself  in  a  very  obvious  contra- 
diction. What  biology  actually  knows  is  nothing  but  an 
uninterrupted  series  of  living  beings,  living  cells,  living 
nuclei,  which  find  a  truthful  expression  in  the  fourfold  law  : 
omne  vivum  ex  vivo ;  omnis  cellula  ex  cellula ;  omnis  nucleus 
ex  nucleo  ;  omne  chromosoma  e  chromosomate.  The  student  of 
nature  must  necessarily  accept  these  laws  as  a  foundation, 
if  he  wishes  to  trace  the  origin  of  life  on  earth,  but  they  will 
carry  him  no  further — they  will  lead  him  round  in  a  circle 
and  never  let  him  see  the  beginning  of  the  mystery.  If,  as 
a  philosopher,  he  wishes  to  study  the  origin  of  life  more  deeply, 
he  is  forced  to  conclude  that  only  some  cause  apart  from  the 
world  could  have  produced  the  first  living  organism  out  of 
matter.  We  have  already  discussed  this  point  in  the  section 
dealing  with  spontaneous  generation  (pp.  204,  &c.).  If  the 
student  of  nature  refuses  to  accept  this  conclusion,  and  is 
resolved  to  be  content  with  what  natural  science  as  such  can 
offer  him,  he  must  simply  say  :  *  We  know  nothing  about  the 
origin  of  life.'  Many  naturalists  of  the  present  day  have 
actually  adopted  this  empirical  standpoint ;  it  was  done, 
for  instance,  by  Branco  in  the  address  that  he  delivered  on 
the  occasion  of  his  admission  to  the  Eoyal  Academy  of  Science 
in  Berlin  ('  Sitzungsberichte  der  Koniglichen  Akademie  der 
Wissenschaften,'  1900,  pp.  679-696). 

What,  then,  are  we  to  think  of  the  theory  of  evolution  ?  It 
certainly  does  not  profess  to  account  for  the  origin  of  organic 
life  on  earth,  it  has  simply  to  accept  it  as  a  fact ;  and  at  the 


same  time  it  accepts  as  a  fact  the  existence  of  laws  governing 
organic  development.  Just  as  philosophical  examination 
has  as  its  necessary  foundation  the  fundamental  principles 
of  thought ;  just  as  no  human  being  can  think  over  any 
philosophical  problem  without  assuming  that  his  understanding 
is  able  to  recognise  the  truth,  that  everything  must  have  a 
sufficient  cause,  and  that  two  contradictory  propositions  cannot 
both  be  true  at  the  same  time  ;  so  no  student  of  nature  can 
consider  theories  of  evolution,  unless  he  assumes  at  the  outset 
as  a  fact  the  existence  of  laws  governing  organic  evolution. 
If  he  refuses  to  admit  that  essentially  the  same  laws  of  organic 
formation,  which  now  govern  the  genesis  of  living  creatures, 
were  in  force  from  the  very  beginning,  he  has  no  clue  at  all  to 
his  phylogenetic  research ;  as  soon  as  he  tries  to  set  aside 
this  fundamental  principle,  his  scientific  investigations  become 
mere  fictions,  with  no  basis  of  fact.  Therefore,  in  considering 
the  race-evolution  of  living  organisms,  we  must  never  lose 
sight  of  the  conclusions  stated  at  the  end  of  Chapters  VI  and 
VIII  (pp.  176,  &c.,  and  pp.  247,  &c.).  In  dealing  with  the 
race-evolution  of  the  living  things  about  us,  we  can  far  less 
dispense  with  internal  laws  of  evolution,  which  are  the  ex- 
pression not  of  a  purely  mechanical,  but  of  a  higher,  vital 
activity,  than  we  can  dispense  with  them  in  dealing  with  the 
phenomena  of  fertilisation,  heredity,  and  ontogeny. 

What  is,  then,  the  real  scope  of  the  doctrine  of  descent,  in 
so  far  as  it  has  a  scientific  basis  ?  Its  task  is,  and  can  only 
be,  to  determine  the  sequence  in  which  the  organic  forms 
appeared  upon  earth,  and  so  to  establish  their  relationship 
with  one  another  ;  it  has,  moreover,  to  investigate  the  causes 
underlying  the  gradual  modifications  in  organic  forms.  The 
task  of  the  theory  of  descent  is,  in  other  words,  to  examine 
the  actual  facts  and  causes  of  the  sequence  of  organic  forms, 
chief  amongst  which  are  the  species  of  the  present  time,  being 
the  last  offshoots  of  one  or  many  hypothetical  pedigrees. 

The  theory  of  evolution  is  not,  and  cannot  be,  an  empirical 
science  (cf.  p.  253  in  §  1),  because  it  is  concerned  with  the 
earliest  history,  antecedent  to  that  of  the  present  organic 
world,  By  collecting  traces  of  that  evolution  from  the  fossil 
records  of  palaeontology  and  by  comparing  them  with  the 
facts  of  the  present,  it  becomes  a  theory  in  natural  science, 


aiming  at  offering  a  probable  explanation  of  the  connexion 
between  these  actual  phenomena. 

From  what  has  been  said  of  the  limitations  of  the  theory 
of  descent,  it  follows  that  it  is  by  no  means  essential  for  it  to 
trace  the  origin  of  all  living  organisms  back  to  one  single  primi- 
tive cell.  Nor  need  it  be  thus  restricted  within  the  limits  of 
the  animal  and  vegetable  kingdoms  respectively,  and  trace  all 
animals  back  to  one  stock,  and  all  vegetables  back  to  another. 
It  is  not  essential  to  the  theory  of  evolution  to  insist  upon  a 
monophyletic  evolution  ;  it  may  just  as  well  decide  in  favour 
of  a  polyphyletic  evolution,  for,  in  examining  the  hypothetical 
race-evolution  of  living  organisms,  it  is  bound  to  conform  to 
facts,  and  not  to  monistic  postulates.  As  I  shall  show  later, 
facts  point  to  a  polyphyletic  evolution  among  both  animals 
and  plants.  Whether  a  monophyletic  or  a  polyphyletic  evo- 
lution is  to  be  accepted  is  therefore,  for  the  scientific  theory 
of  descent,  a  question  of  fact  and  not  of  principle. 

From  this  we  may  deduce  two  statements  that  are  important 
in  our  investigation  :  1st.  The  extreme  champions  of  the 
theory  of  descent,  who  recognise  only  a  monophyletic  evolution 
as  the  real  theory  of  descent,  and  reject  polyphyletic  evolution 
as  being  merely  the  theory  of  permanence  in  disguise,  are 
influenced  by  monistic  prejudices  and  not  by  a  genuinely 
scientific  spirit ;  l  they  completely  misunderstand  what  the 
scientific  doctrine  of  evolution  really  is.  2nd.  Equally  mis- 
taken is  the  attitude  of  those  opponents  of  the  theory  of 
descent,  who  try  to  prove  that  the  whole  doctrine  of  evolution 
has  broken  down,  because  no  one  has  yet  succeeded,  and  prob- 
ably no  one  ever  will  succeed,  in  tracing  back  the  chief  types 
of  the  animal  and  vegetable  kingdoms  to  one  single  stock. 
I  cannot  therefore  concur  with  Fleischmann's  opinions,  ex- 
pressed in  his  book  '  Die  Deszendenztheorie  '  (Leipzig,  1901). 
In  many  passages  he  bases  his  arguments  against  the  theory 
of  descent  on  the  statement  that  the  types  of  organisation  among 
animals  cannot  phylogenetically  be  derived  from  one  single 
type.  This  proves  nothing  but  that  polyphyletic  evolution 
must  be  accepted  rather  than  monophyletic  ;  it  does  not 

1 1  wish  this  remark  to  be  taken  to  heart  by  Escherich,  Forel,  Haeckel,  von 
Wagner  and  others,  who  criticised  my  first  edition.  See  also  '  A  Few  Words 
to  my  Critics,'  at  the  beginning  of  this  volume. 


prove  that  an  evolution  of  the  species  within  definite  series 
of  forms  or  genera  is  impossible.  Arguments  of  this  kind 
affect  only  the  monistic,  and  not  the  scientific,  theory  of 
descent.  In  general,  I  am  unable  to  share  Fleischmann's 
views,  which  are  involved  in  pure  empiricism  and  agnosticism. 


'  But/  some  one  may  say,  '  why  do  we  not  simply  assume 
that  the  species  in  the  world  of  organic  life  have  always  been 
what  they  are  at  the  present  day  ?  Why  do  we  want  any 
theory  of  evolution  at  all  ?  ' 

I  am  bound  to  explain  this  point  to  my  readers,  at  least 
to  some  extent,  before  I  go  on  to  discuss  the  modern  theory  of 
descent  more  in  detail.  Three  hundred  and  fifty  years  ago, 
when  war  broke  out  between  the  old  Ptolemaic  view  of  the 
universe  and  the  new  Copernican  view,  people  had  no  con- 
ception of  the  distance  to  which  they  would  be  carried  by  the 
ideas  that  then  took  possession  of  the  human  intellect.  It  was 
not  until  the  nineteenth  century,  that  from  the  heliocentric 
theory  of  the  universe  inferences  were  made  affecting  the 
natural  development  of  our  solar  system,  and  the  whole  history 
of  the  universe,  of  which  the  geological  development  of  our 
earth  occupies  but  an  insignificant  moment  of  time.  And 
within  this  insignificant  period  (which,  in  comparison  with  the 
development  of  the  whole  universe,  is  like  a  second  between 
two  eternities,  although  according  to  geologists  it  really  lasted 
millions  of  years)  is  another  period  of  history  preceding  that 
in  which  man  appeared  upon  the  world,  and  this  is  the  history 
of  the  animal  and  vegetable  kingdoms  from  the  earliest 
palaeozoic  age  until  the  present  time. 

The  Copernican  system  revealed  to  us  the  earth  as  a  mere 
atom  in  the  universe,  as  one  of  the  many  planets  attendant 
upon  a  central  sphere,  that  we  call  the  sun.  But  our  sun  is 
not  the  only  sun  ;  there  are  thousands  of  others,  many  being 
still  far  larger  than  it  is.  All  the  innumerable  fixed  stars  that 
we  see  in  the  sky  at  night  are  so  many  suns,  which  are  not, 
however,  scattered  at  random  in  space,  but  form  one  single 
huge  cosmic  system.  This  system  is  not  an  unalterable 


mathematical  formula  in  its  various  components.  Astronomy 
teaches  us  that  the  constellations  are,  at  different  stages  in 
their  evolution,  ranging  from  gaseous  vapour  to  molten  matter 
like  the  sun,  and  even  to  the  dark  planets,  that  are  visible 
only  by  the  light  of  others. 

This  is  where  the  theories  of  Kant  and  Laplace  on  cos- 
mogony find  their  points  d'appui  ;  they  strive  to  account  for 
the  genesis  of  the  whole  universe  by  one  uniform  law.1  By 
means  of  the  laws  which  now  control  the  movements  and 
conditions  of  the  celestial  bodies,  this  cosmogony  seeks  to 
ascertain  how  our  solar  system,  and  the  cosmic  system  as  a 
whole,  assumed  their  present  form.  It  was  led  to  accept 
the  existence  of  an  original  enormous  sphere  of  gas,  in  which, 
as  it  gradually  cooled  and  condensed,  a  rotatory  movement 
arose,  that  caused  the  formation  of  the  solar  systems.  Accord- 
ing to  the  same  cosmic  laws,  the  planets  subsequently  separated 
from  each  sun,  in  order  to  circle  round  it  on  definite  paths. 
And  one  of  these  planets  is  our  earth.  Many  modifications 
have  recently  been  introduced  into  the  theories  that  are  called 
after  Kant  and  Laplace,3  but  it  is  not  likely  that  any  new 
theory  will  take  its  place,  at  least  as  far  as  its  essential 
outlines  go. 

T.  C.  Chamberlin's  '  Spiral  Nebulae  Theory  '  3  suggests 
a  different  explanation  for  the  origin  of  the  planetary  system 
of  a  sun,  but  still  it  presupposes  the  existence  of  the  gaseous 

No  matter  what  scientific  form  the  theories  regarding 
cosmogony  may  take,  their  problem  is  always  to  account  for 
the  present  form  and  arrangement  of  the  heavenly  bodies, 
and  to  explain  how  this  form  and  arrangement  may  have 
been  evolved  by  natural  means. 

At  the  present  day  there  are  probably  very  few  who  still 
cling  to  the  old  theory  that  sun,  moon,  earth,  planets  and 

1  Cf.  J.  Epping,  S.J.,  Der  Kreislauf  im  Kosmos,  Freiburg,  1882  (Supplement 
to  Stimmen  aus  Maria-Laach,  Part  18} ;  also  an  excellent  work  by  K.  Braun,  S.  J., 
Uber  Kosmogonie  vom  Standpunkt  christlicher  Wissenschaft,  Miinster,  1905. 

2  The  theories  of  Kant  and  Laplace  on  cosmogony  are  somewhat  different, 
and  cannot  be  united  under  one  name,  as  Stolzle,  Gockel,  and  other  recent 
authors    have    shown.     See    A.    Gockel,    Schopfungsgeschichtliche    Theorien. 
Cologne,  1907. 

3  Cf.  F.  R.  Moultpn,  'The  Evolution  of  the  Solar  System'  (Astrophysical 
Journal,  XXII,  1905,  pp.  165-181).     See  also  the  review  in  the  Naturwissen- 
schajtliche,  Rundschau,  1906,  No  5,  pp.  53-56. 


all  the  fixed  stars  in  the  universe  were  created  once  for  all  as 
we  now  know  them.  Even  to  St.  Augustine  it  seemed  a  more 
exalted  conception,  and  one  more  in  keeping  with  the  omni- 
potence and  wisdom  of  an  infinite  Creator,  to  believe  that 
God  created  matter  by  one  act  of  creation,  and  then  allowed 
the  whole  universe  to  develop  automatically  by  means  of 
the  laws  which  He  imposed  upon  the  nature  of  matter. 

God  does  not  interfere  directly  with  the  natural  order 
when  He  can  work  by  natural  causes  :  this  is  a  fundamental 
principle  in  the  Christian  account  of  nature,  and  was  enunciated 
by  the  great  theologian  Suarez,1  whilst  St.  Thojnas_Aq[uinas 
plainly  suggested  it  long  before,  when  he~regarded  it  as  testi- 
mony to  the  greatness  of  God's  power,  that  His  providence 
accomplishes  its  aims  in  nature  not  directly,  but  by  means 
of  created  causes.2 

Is  it  not  reasonable  for  us  to  try  to  apply  the  same  principle 
of  independent  evolution  also  to  the  living  creatures  that 
inhabit  our  globe  ?  The  obvious  complement  to  the  geological 
history  of  our  world  is  the  history  of  the  creatures  that  have 
dwelt  on  it,  since  the  time  when  organic  life  first  made  its 
appearance.  In  the  geological  arrangement  of  strata  we  see 
the  working  of  natural  forces  influencing  the  formation  of 
the  earth's  surface,  and,  in  the  same  way,  in  the  fossil  animals 
and  plants  we  see  the  remains  of  organisms  that  really  lived 
at  those  respective  epochs.3 

Palaeontology  teaches  us  that  our  present  species  of 
animals  and  plants  have  not  always  existed.  It  shows  us 
that  there  was  a  succession  of  different  organic  forms  in  the 

1  De  opere  sex  dierum,  1.  2,  c.  10,  n.  12.     Further  evidence  to  show  that  this 
idea  was  by  no  means  strange  to  St.  Augustine,  St.  Thomas,  St.  Bonaventure 
and  others  may  be  found  in  Father  Knabenbauer's   '  Glaube  und  Deszendenz- 
theorie  '  (Stimmen  aus  Maria-Laach,  XIII,  1877,  pp.  75,  &c.).  Cf.  also  T.  Pesch, 
Philosophianaturalis,  II,  pp.  241,  &c.,  and  Die  grossen  Weltrdtsel,  II,  pp.  349,  &c. 

2  Summa  c.  gent.,  1.  3,  c.  77. 

3  The  idea  that  fossils  were  originally  created  as  such,  and  represent  mere 
lusus  naturae,  is  just  as  groundless  as  the  other  opinion,  that  all  fossils  date 
from  the  deluge.     The  first  idea  is  wrong  in  principle,  and  contradicts  the 
fundamental  laws  of  all  intelligent  research ;  it  is  opposed,  therefore,  to  the 
true  philosophy  of  nature,  and  leads  inevitably  to  occasionalism,  and  is  equi- 
valent to  a  complete  abandonment  of  all  hope  of  giving  a  natural  account  of 
palseontological  facts.     The  second  theory  may  not  be  intended  to  clash  with 
geology  and  palaeontology,  but  it  is  manifestly  wrong  in  assuming  that  all  the 
strata  containing  fossils,  more  than  20,000  in  number,  can  be  accounted  for 
by  the  deluge. 


various  geological  periods,  and  in  this  succession  the  species 
of  animals  and  plants  that  appeared  later  approximated  more 
and  more  closely  to  those  of  the  present  time,  and  in  many 
cases — e.g.  in  the  extinct  connexions  of  the  horse — the 
succession  suggests  upward  lines  of  evolution,1  and  our  present 
species  are  their  latest  developments.2 

We  have  now  to  face  the  critical  question :  '  Does  this 
gradual  or  more  abrupt  approximation  of  the  fossil  Fauna 
and  Flora  to  those  of  the  present  depend  upon  a  mere  succession 
of  forms,  constantly  becoming  more  like  the  present  forms, 
or  is  it  a  real  evolution,  a  genetic  production  of  various 
systematic  species  from  one  another  ?  Are  these  "  evolutionary 
series,"  which  lead  us  from  fossil  ancestors  to  now  existent 
species,  merely  apparent  ?  Do  they  owe  their  origin  to  the 
fact  that,  at  the  close  of  the  various  geological  formations 
and  groups  of  formations,  a  great  catastrophe  occurred,  destroy- 
ing all  living  creatures,  which  at  the  beginning  of  the  next 
period  were  replaced,  by  means  of  a  new  creation,  by  similar 
creatures,  for  the  most  part  somewhat  more  highly  organised  ? 
Or  are  these  evolutionary  series  real  and  natural,  depending 
upon  a  genealogical  connexion  between  the  organisms  of 
various  periods  ?  ' 

There  can  scarcely  be  any  doubt  as  to  the  answer.  Cuvier's 
theory  of  a  catastrophe  has  been  given  up  by  geologists,  because, 
when  generalised,  it  proved  to  be  inconsistent  with  facts  ; 
consequently  it  had  to  be  given  up  by  palaeontologists  also, 
In  place,  therefore,  of  the  periodically  repeated  '  new  creations/ 
the  theory  of  a  natural  evolution  of  organic  forms  has  won 

1  The  hypothetical  pedigree  of  the  Equidae  does  not,  however,  form  a  simple 
line  of  evolution,  but  it  has  many  ramifications,  and  since  the  Lower  Eocene 
age  they  have  developed  on  distinct  lines  in  Europe  and  North  America.     Cf. 
Zittel,  Grundziige  der  Paldontologie  (Munich  and  Leipzig,  1895),  p.  871. 

2  I  am  not,  however,  speaking  here  of  evolutionary  series  in  the  sense  of 
Darwin's  theory  of  transmutation,  i.e.  not  of  series  of  very  small  and  gradual 
transitions,  for  these,  if  they  occur  at  all,  are  an  exception  to  the  more  usual 
transitions  *  by  steps,'  that  involve  greater  changes.     Hilgendorf's  famous 
Planorbis  series  has  proved  not  to  be  a  progressive  sequence  of  variations, 
but  rather  a  cycle  of  recurring  variations,  and  it  is  of  no  use  for  the  purposes 
of  phylogeny.     (Cf.  K.  Miller,  '  Die  Schneckenfauna  des  Steinheimer  Obermio- 
cans,'  in  the  Jahreshefte  fur  vaterldndische  Naturkunde  in  Wurttemberg,  1900, 
pp.  385-406  with  Plate  VII.)  L.  Doderlein's  dictum  (Zeitschrift  fur  Morphologie 
und  Anthropologie,  IV,  1902,  Part  2,  p.  408)  that  complete  knowledge  of  any 
group  of  animals  requires  all  the  forms  in  that  group  to  stand  in  unbroken 
sequence,  is  not  based  on  fact,  but  is  a  theoretical  postulate  of  the  Darwinian 
theory  of  evolution. 



acceptance,1  in  logical  application  of  the  principle  that  God 
does  not  interfere  directly  with  the  natural  order,  when  He 
can  work  by  natural  causes. 

The  theory  of  evolution,  regarded  without  prejudice, 
is  then  for  us  the  latest  outcome  of  the  Copernican  theory  of 
the  universe,  which  no  one  probably,  at  the  present  day,  will 
call  un-Christian. 

A  few  instances  may  be  added  by  way  of  illustration. 
If  we  find  the  Brachiopod  Order  Lingula  occurring  frequently 
in  the  Silurian  and  Devonian  strata,  and  continuing  to  appear 
at  different  geological  epochs  in  various  species  down  to  the 
present  day,  we  must  undoubtedly  say  :  '  The  modern  species 
of  Lingula  are  really  connected  with  those  of  the  Silurian  age  ; 
in  fact  they  are  their  modified  descendants.'  If  in  the 
Cambrian,  the  oldest  strata  containing  any  fossils,  we  find 
representatives  of  the  family  of  Nautiloidea,  various  genera 
and  species  of  which  still  exist,  we  must  say  in  the  same  way  : 
*  The  still  existing  four  species  of  Nautilus  are  modified  descen- 
dants of  members  of  the  same  family  belonging  to  earlier 
ages  of  the  world.'  If  we  compare  our  crickets  (Phasmidae) 
with  those  of  the  Carboniferous  period,  we  shall  be  forced 
to  ascribe  to  them  not  merely  a  theoretical,  but  a  real  relation- 
ship with  the  Protophasma  and  the  Titanophasma  of  the  coal 
age.  If  we  compare  our  ants  and  Paussidae  with  those  found 
in  Baltic  amber  of  the  Tertiary  period,  we  cannot  possibly  think 
that  they  are  new  creations,  but  must  regard  them  as  genuine 
descendants  of  the  Tertiary  forms,  although  differing  from 
them  partly  specifically  and  partly  generically.  Any  other 
view  of  the  matter  seems  scientifically  almost  impossible. 

If  we  compare  fossil  termites 2  with  those  of  the  present 

1  It  is  a  remarkable  fact  that  more  than  two  hundred  years  ago,  a  famous 
Jesuit,  Father  Athanasius  Kircher,  in  his  book,  Area  Noe  in  tres  libros  digesta 
(Amsterdam,  1675),  expressed  his  belief  that  our  modern  species  had  originated 
by  transmutation  within  definite  series  of  forms.     (On  this  subject  see  Daniele 
Rosa,  « II  Rev.  Padre  Kircher  trasformista,'  Bolletino  dei  Musei  di  Zoologia 
ed  Anatomia  comparata  d.  R.  Universith  di  Torina,  XVI,  No.  421,  March  14, 
1902.)     Although  Father  Kircher's  views  were  based  on  insufficient  data,  we 
are  all  the  more  justified  in  holding  similar  opinions,  as  our  scientific  knowledge 
is  much  greater. 

2  According   to    Handlirsch,    remains   of   termites   occur   with   certainty 
only  after  the  early  Tertiary  period ;    he  does  not  regard  as  termites  what 
Heer  described  as  such  occurring  in  the  Black  Jurassic  strata.     His  views, 
however,  do  not  in  any  way  affect  the  above  statement  respecting  the  connexion 
between  our  present  termites  and  those  of  the  Tertiary  period. 


day,  we  cannot  doubt  that  they  all  form  one  single  natural 
stock,  continuing  from  the  Mesozoic  group  of  formations 
through  the  Ccenozoic  to  the  Alluvial  present.  The  extinct 
fossil  genus  Clathrotermes,  of  the  black  Lias,  represents  one 
natural  stock  with  the  fossil  varieties  of  the  genus  Calotermes, 
belonging  to  the  same  period.  Of  this  latter  genus  many 
species  still  exist,  which  differ,  however,  from  those  occurring 
in  the  lias.  With  regard  to  the  much  greater  variety  of  fossil 
termites  of  the  Tertiary  period,  which  include  a  great  many 
still  existent  genera  and  one  that  is  extinct  (Parotermes), 
we  cannot  question  the  fact  that  they  are  genetically  connected 
both  with  the  termites  of  the  Lias  and  with  those  of  the  present 
day,  although  their  species  are  different  both  from  the  Mesozoic 
and  the  modern.  We  still  find  in  Australia  a  curious  genus  of 
termites,  Mastotermes,  whose  wing- veins,  in  my  opinion,  show 
that  they  are  unmistakably  connected  with  the  Palaeozoic 
Blattinae  of  the  coal  age  ;  and  this  fact  justifies  our  assuming 
that  we  have  in  Mastotermes  the  last  living  representative  of 
the  oldest  and  most  original  form  of  termite,  which  as  a 
'  collective  type  ' l  has  united  in  itself  the  venous  systems  of 
cockroaches  and  termites,  that  afterwards  became  entirely 
distinct.  Australia  is  particularly  rich  in  old  forms,  which 
occur  in  other  parts  of  the  world  only  in  a  few  still  surviving 
representatives,  or  as  fossils  dating  from  earlier  ages.2  These 
instances  are  quite  enough  to  prove  that  it  is  hardly  possible 
to  deny  the  existence  of  a  genuine  race-connexion  between 
our  modern  forms  of  animals  and  the  extinct  species  of  bygone 
ages.  We  may  now  return  to  our  consideration  of  the  doctrine 
of  evolution. 

Under  Haeckel's  guidance,  the  monists  have  misused  the 

1  Forms  which  show  the  characteristics  of  several  systematic    groups  are 
called    '  collective    types.'       Such,    for    instance,    is    Peripatus    among    the 
Arthropods,  which  by  its  low  organisation  approaches  the  Annelids.     According 
to  Handlirsch  (VerhandL  d.  Zool.  Bot.  Gesellsch.,  Vienna,  1906,  Part  3,  p.  91), 
it  ought  to  be  classed  among  the  Annelids.     Numerous  collective  types  occur, 
especially  among  the  palaeozoic  insects,  to  which   Skudder  gives  the  general 
name  of  Palaeodictyoptera. 

2  In  support  of  this  statement  I  may  refer  to  the  Monotremata  and  Mar- 
supials among  mammals,  and    to  the  genus  Arthropterus  in   the  family  of 
Paussidae.     Australia  seems  to  have  preserved  the  oldest  type  of  the  human 
race,  for  Macnamara  has  recently  shown  that  the  cranial  formation  of  modern 
Australian  and  Tasmanian  blacks  approximates  very  closely  to  that  of  the 
fossil  Neandertal  man.     We  shall  come  back  to  Macnamara's  statements  in 
Chapter  XI. 


theory  of  evolution,  and  by  making  it  serve  as  a  weapon  with 
which  to  attack  the  theism  that  they  hate,  they  have  brought 
it  into  disrepute  in  conservative  circles  ;  and  so  the  idea  has 
arisen  that  the  theory  of  evolution  is  an  absolutely  atheistical 
device,  directly  opposed  to  Christianity.  I  have  just  shown 
this  idea  to  be  erroneous,  and  to  have  no  foundation.  If  we 
wish  successfully  to  combat  the  modern  theory  of  descent, 
in  so  far  as  it  has  proved  serviceable  to  atheism,  we  must 
carefully  distinguish  truth  and  falsehood  in  it.  We  shall 
then  have  no  difficulty  in  depriving  our  antagonists  of  their 
weapons,  and  even  in  smiting  them  with  the  same  sword  with 
which  they  fancied  we  were  already  conquered.  If  we  let 
ourselves  be  misled  by  the  skilful  tactics  of  our  monistic 
opponents,  and  take  up  an  attitude  hostile  to  evolution  in 
every  form,  we  shall  be  playing  into  their  hands  and  giving 
them  an  easy  victory.  We  shall  in  fact  be  assuming  the 
same  mistaken  position  as  the  champions  of  the  Ptolemaic 
system  once  assumed  against  the  advocates  of  the  Copernican 
theory.  They  were  obliged  to  be  always  on  the  defensive,  and 
to  limit  themselves  to  weakening  this  or  that  actual  piece  of 
evidence  adduced  by  their  opponents,  as  not  holding  good. 
In  an  intellectual  conflict  such  a  position  must,  in  course  of 
time,  be  abandoned.  A  succession  of  retreats  brings  the 
defenders  on  to  more  and  more  dangerous  ground,  and  finally 
leads  to  a  decisive  defeat.  If  Christianity  is  not  to  succumb 
to  the  attacks  of  monism  based  on  natural  philosophy,  it  must 
determine  upon  bold  action  on  the  offensive ;  it  must 
seize  the  enemies'  arsenal,  and  by  accepting  without  reserve 
whatever  is  right  in  the  theory  of  evolution,  it  will  turn  its 
opponents'  weapons  against  themselves.  In  such  proceedings 
caution  is  always  advisable.  Not  everything  alleged  by  the 
supporters  of  the  theory  of  descent  to  be  *  based  on  actual 
facts  '  really  deserves  belief.  I  need  only  remind  my  readers 
of  Haeckel's  famous  pedigree  of  man,  of  which  one  critic 
sarcastically  remarked  that  it  was  just  as  worthy  of  credence 
as  those  of  the  Homeric  heroes.  We  must  examine  carefully 
how  far  we  can  accept  the  ideas  involved  in  the  theory  of 
evolution,  both  from  the  philosophical  and  the  scientific 
points  of  view.  There  must  be  no  mention  of  concessions. 
We  make  concessions  to  error  only  when,  through  cowardice 


or  weakness,  we  accept  what  is  wrong  as  right,  or  what  is  half 
true  as  quite  true  ;  but  it  is  not  a  concession  when  we  deprive 
error  of  the  weapons  that  it  is  using  in  the  struggle  against  truth. 


1.  What  are  we,  therefore,  to  think  of  the  theory  of  descent 
in  its  relation  to  philosophy  ?  It  has  already  been  shown 
that  the  acceptation  of  an  evolution  of  the  organic  species  is 
only  a  logical  consequence  of  the  cosmic  and  geological  evolu- 
tion. On  the  philosophical  side  it  would  be  possible  to  reject 
the  theory  of  descent  only  if  it  could  be  proved,  on  purely 
philosophical  grounds,  that  our  present  species  are  absolutely 
unchangeable,  and  that  therefore  there  can  be  no  question 
of  their  having  evolved  from  older  forms.  But  philosophy 
cannot  adduce  any  proof  of  this  kind,  because  the  subject 
does  not  fall  within  her  scope.  She  is  obliged  to  leave  natural 
science  to  decide  whether  the  systematic  species  are  altogether 
constant  magnitudes  or  not,  and  we  have  already  seen  what 
this  decision  is,  and  shall  refer  to  it  again  later. 

The  fundamental  principle  laid  down  by  philosophy  with 
reference  to  the  theory  of  evolution  agrees  perfectly  with 
Christianity,  and  may  be  stated  thus  :  '  It  is  not  permissible 
to  assume  any  immediate  interference  on  the  part  of  the 
Creator,  where  the  facts  can  be  explained  by  natural  evolution.' 
In  applying  this  principle  we  must  be  careful  to  distinguish  the 
philosophical  and  the  scientific  standpoints.  Many  things 
are  possible  in  themselves,  and  even  probable,  a  priori,  but 
there  is  no  scientific  proof  of  their  occurrence.  The  limits 
assigned  to  us  by  philosophy,  with  regard  to  our  acceptance 
of  the  theory  of  evolution,  are  far  wider  than  those  imposed 
upon  us  by  natural  research  as  to  details.  Moreover,  the 
former  are  fixed  and  cannot  be  overthrown  ;  the  latter  are 
constantly  changing  as  our  positive  knowledge  advances. 
We  must,  therefore,  carefully  distinguish  between  the  limits 
set  by  philosophy  and  natural  science  respectively  to  the 
theory  of  evolution  ;  and,  in  dealing  with  the  philosophical 
limits,  we  must  again  distinguish  between  purely  philosophical 
questions  and  those  that  are  of  a  mixed  character. 


Let  us  first  consider  the  philosophical  limits.  In  one 
sense  philosophy  has  only  to  sketch  the  hroad  outlines  of  the 
theory  of  evolution ;  it  is  the  task  of  natural  science  to  fill 
in  the  details.  The  first  and  foremost  boundary,  admitting 
of  no  modification  whatever,  is  the  principle  that  the  hypo- 
thetical race-evolution  of  the  organic  species  must  have  had 
an  adequate  first  cause.  This  principle  contains  two  postulates, 
one  purely  philosophical,  and  one  partly  philosophical  and 
partly  belonging  to  natural  science.  The  first  is  :  '  We  must 
assume  the  existence  of  a  personal,  all-wise  and  all-powerful 
Creator  as  the  first  cause,  extraneous  to  the  world,  of  the 
whole  cosmos  and  its  laws  of  evolution/  The  second  is  :  *  In 
order  to  account  for  the  origin  of  the  first  organisms,  we  must 
accept  some  special  action,  direct  or  indirect,  on  the  part  of 
the  Creator  upon  matter.'  Here  we  are  not  concerned  with 
1  Creation,'  strictly  speaking,  as  we  are  in  the  first  postulate, 
but  only  with  the  production  of  the  primitive  organisms 
from  already  existent  inorganic  matter,  which  had  been  formed 
by  a  definite  act  of  creation  at  the  beginning  of  the  cosmic 
evolution.1  The  formation  of  the  first  living  creatures  followed, 
therefore,  by  an  eductio  formarum  e  potentia  materiae,  as  scholastic 
philosophy  expressed  it.  As  intelligent  beings  we  cannot 
dispense  with  this  postulate  ;  all  the  efforts  of  monism  to  set  it 
aside  are  fruitless.  This  postulate  is  of  a  mixed  character, 
not  purely  philosophical  like  that  regarding  the  creation  of 
primitive  matter,  for  natural  science  proves  that  an  essential 
difference  exists  between  animate  and  inanimate  substances, 
and  shows  us  the  absolute  incompatibility  of  the  laws  of  biology 
and  the  theory  of  spontaneous  generation.  (Cf.  Chapter  VII, 
pp.  198,  &c.)  Neither  philosophy  nor  natural  science  can  tell 
us  how  the  first  organisms  came  into  being ;  no  facts  that 
we  can  observe  enable  us  to  infer  anything  on  this  subject. 
Nor  can  philosophy  say  how  many  primitive  organisms  were 
produced,  and  whether  they  differed  essentially  from  one 
another  or  not.  Yet  a  somewhat  important  limitation  seems 
to  meet  us  here.  As  sensitive  life  is  on  a  higher  level  than 
vegetative,  it  is  reasonable  to  suppose  that  the  former  could 

1  I  wish  to  draw  particular  attention  to  this  passage,  as  some  of  the  critics 
of  my  previous  edition  fell  into  the  error  of  regarding  the  creation  of  the 
first  organisms  as  a  creatio  e  nihilo. 


not  have  evolved  itself  out  of  the  latter.  We  must  therefore 
assume  that  when  organic  forms  first  came  into  being,  there 
was  in  all  probability  a  differentiation  among  them  into 
animals  and  vegetables.  This  postulate  is  of  a  mixed  character, 
partly  philosophical  and  partly  scientific,  and  is  by  no  means 
absolutely  fixed.  For,  on  the  one  hand,  while  observed  facts 
show  us  the  great  difference  between  the  vegetative  and 
sensitive  life  of  the  higher  animals  and  the  merely  vegetative 
life  of  the  higher  plants  ;  on  the  other  hand,  they  reveal 
to  us  a  number  of  unicellular  organisms,  which  zoologists 
reckon  among  the  lower  animals,  and  botanists  among  the 
lower  plants  ; l  and  in  their  case  it  is  impossible  to  say  whether 
the  sensitiveness  of  the  protoplasm,  which  is  a  general  character- 
istic of  all  living  cells>  amounts  to  real  sensation  or  not.2  We 
have  also  to  take  into  consideration  the  movements  made  by 
certain  plants  in  response  to  external  stimulus,  for  which  a 
purely  vegetative  interpretation  seems  inadequate,3  although 
I  agree  with  J.  Keinke 4  in  thinking  that  the  so-called  '  sense 
organs  '  of  plants  represent  merely  the  receptive  centres  for 
physical  and  chemical  stimuli,  and  we  are  not  justified  in  arguing 
from  them  that  plants  have  sense  perception.  We  probably 
ought  not  to  regard  the  original  difference  of  animal  and 
vegetable  organisms  as  an  unalterable  philosophical  postulate  ; 

1  On  this  subject  see  von  Wettstein,  Handbuch  der  systematischen  Botanik, 
I,  1901,  pp.  16,  &c. 

2  We  derive  our  ideas  of  plants  and  animals  from  the  higher  varieties 
of  both,  which  it  is  perfectly  easy  to  distinguish,  but  there  are  obviously  great 
difficulties  in  applying  these  ideas  to  unicellular  organisms. 

3  Cf.  Haberlandt,  Die  Sinnesorgane  im  Pflanzenreich  zur  Perzeption  mechani- 
scher  Reize,  Leipzig,  1900  ;     '  Die  Sinnesorgane  der  Pflanzen  '  (paper  read  at 
the  seventy-sixth  meeting  of  German  naturalists  at  Breslau,  September  23, 
1904,  published  in  the  Naturwissenschaftliche  Rundschau,  1904,  Nos.  45  and 
46) ;    '  tiber  den  Begriff  "  Sinnesorgan"  in  der  Tier-  und  Pflanzenphysiologie  ' 
(Biologisches  Zentralblatt,  1905,  No.  13,  pp.  446-451 ) ;   '  Die  Lichtsinnesorgane 
der  Laubblatter '  (ibid.,  No.  17,  pp.  580-588).    See  also  Fr.  Noll,  Das  Sinnesleben 
der  Pflanzen,  Frankfurt  a.  M.,  1896  ;  F.  R.  Schrammen,  '  Kritische  Analyse  von 
G.  Th.   Fechners  Werk  :    Nanna  oder  iiber  das  Seelenleben  der  Pflanzen  ' 
(Verhandl.  des  Naturhist.  Vereins,  Bonn,  LV,  1903,  pp.  133-199).     On  p.  198 
Schrammen  seems  to  think  that  we  ought  to  ascribe  to  plants  a  sensitive, 
but  not  an  intelligent  existence.     This  is  intelligible  only  if  he  means  by  a 
sensitive  existence  merely  susceptibility  to  mechanical  and  other  stimuli, 
not  amounting  to  perception.     Many  botanists  speak  of  plants  as  sensitive 
to  light,  but  the  word  is  then  used  inaccurately,  as  it  is  when  photographic 
paper  is  so  described.    It  is  not  possible  in  either  case  to  use  the  word  '  sensitive  ' 
in  its  strict  psychological  meaning ;    we  ought  rather  to  say  susceptible  to 

4  Philosophie  der  Botanik,  1905,  pp.  83-87  and  113. 


that  the  whole  organic  world  may  have  been  evolved  from  one 
single  primitive  cell  is  not  an  absolute  impossibility,  though 
it  is  improbable.  This  improbability  appears  greater  when  we 
take  into  account  the  important  physiological  distinction 
between  the  two  kingdoms,  which  0.  Hertwig  ('Allgemeine 
Biologie/  1906,  p.  602)  states  as  follows :  *  In  consequence 
of  their  characteristic  metabolism,  the  whole  formation  of 
chlorophyll-bearing  plants  is  directed  towards  the  exterior 
and  is  visible  from  the  exterior,  but,  unlike  animal  organisms, 
plants  either  show  no  interior  differentiation  into  organs 
and  tissues,  or  they  show  it  in  a  relatively  limited  degree.' 

Philosophy  can  give  us  no  information  at  all  regarding 
the  number  of  forms  of  plants  and  animals  originally  produced, 
nor  can  it  tell  us  whether  they  were  produced  once  for  all  and 
in  one  place,  or  on  many  occasions  and  in  various  places. 
Natural  science,  too,  in  the  present  state  of  our  knowledge, 
can  throw  very  scanty  light  upon  this  subject,  but  I  shall 
return  to  this  topic  later  ;  let  us  now  consider  it  only  from 
the  point  of  view  of  philosophy. 

Philosophy  is  not  concerned  to  decide  whether  the  animal 
world  on  the  one  hand,  and  the  vegetable  world  on  the  other 
hand,  were  each  descended  from  one  primitive  form  (mono- 
phyletic  evolution),  or  whether  they  originated  simultaneously 
or  successively  from  several  primitive  forms,  independent  of 
one  another  (polyphyletic  evolution).  Nor  does  philosophy 
tell  us  anything  of  the  causes  that  motive  race-evolution  ; 
however,  the  fact  that,  as  natural  science  shows  us,  at  the 
present  time  interior  laws  of  development  are  the  ultimate 
foundation  of  all  organic  genesis,1  justifies  philosophy  in 
inferring  that  the  race-evolution  of  living  organisms  chiefly 
and  essentially  must  have  been  the  result  of  interior  causes 
of  development.  All  the  exterior  causes  are  simply  aimless, 
unless  we  presuppose  the  existence  on  the  part  of  the  organism 
of  a  corresponding  interior  capacity  for  development ;  and 
this  capacity  must  ultimately  have  been  implanted  by  the 
Creator  in  the  nature  of  the  primitive  types.  Therefore 
philosophy  is  justified  in  drawing  the  further  inference  that 

1  Some  suggestions  respecting  the  probable  material  bearers  of  these  laws 
of  development  were  made  in  Chapter  VI,  pp.  164,  &c.  Cf.  also  the  conclusion 
of  Chapter  VIII. 


those  theories  of  descent,  which  attach  the  utmost  importance 
to  the  exterior  causes  of  development,  whilst  underrating 
the  interior,  must  be  regarded  as  unsatisfactory.  Thus  far 
philosophy  may  and  must  utter  her  decisions  ;  but  in  herself 
she  can  tell  us  nothing  as  to  the  character  of  these  interior 
causes  of  development  and  how  they  co-operate  with  the 
exterior  factors  ;  any  knowledge  that  she  possesses  on  these 
points  is  borrowed  from  natural  science. 

She  does  not  inform  us  whether  a  race-evolution  of  the 
organic  species  ever  really  took  place  or  not ;  she  does  not 
tell  us  anything  as  to  the  number  of  the  original  primitive  forms  ; 
she  teaches  us  nothing  about  the  laws  governing  the  hypotheti- 
cal race-evolution  of  organisms,  nor  the  order  in  which  it  took 
place.  If  she  is  wise,  she  will  leave  it  to  natural  science  to 
express  an  opinion  on  these  points  ;  l  but  there  is  one  thing 
of  great  importance,  which  she  is  able  to  tell  us.  Without 
a  first  cause  outside  the  world,  the  existence  of  matter  and 
the  laws  governing  its  development  would  have  been  im- 
possible ;  without  the  same  first  cause  outside  the  world, 
the  origin  of  living  organisms  from  inorganic  matter  would 
have  been  inconceivable,  and  consequently  a  race-evolution 
of  the  organic  world  would  have  been  out  of  the  question; 
and,  in  exactly  the  same  way,  we  can  account  for  the  existence 
of  man  only  by  assuming  some  special  action  on  the  part  of 
the  Creator,  this  special  action  being  the  creation  of  the  human 

1  The  writer  of  a  review  on  the  first  edition  (in  the  Innsbrucker  Zeitschr.  fur 
katholische  Theologie,  1905,  p.  561),  asks  :  '  Is  there  philosophically  no  diffi- 
culty in  assuming  that  the  sparrow  and  the  hippopotamus  have  branched 
off  from  the  same  primitive  form  ?  Are  we  not  forced  to  believe  that  there 
is  an  essential  difference  in  their  inner  nature,  in  their  very  soul  ?  '  I  do  not 
think  that  this  question  admits  of  a  purely  philosophical  answer.  If  it  were 
worded  scientifically,  it  would  be  simply :  '  Are  birds  and  mammals  to  be 
regarded  as  related  ?  '  On  examining  the  scientific  limitations  of  the  evolution 
theory  we  shall  find  that  there  is  very  little  to  be  said  in  support  of  the  common 
descent  of  all  vertebrates.  Moreover,  as  mammals  appear  in  the  Triassic,  and 
birds  only  in  the  Jurassic  strata,  there  are  no  intermediate  forms  between 
birds  and  mammals.  It  is  true  that  in  some  respects  our  present  Monotremata 
(Ornithorhynchus  and  Australian  ant-eating  Echidna)  occupy  a  position 
midway  between  these  two  classes  of  vertebrates,  but  in  other  respects  there 
are  important  differences.  (Cf.  Fleischmann,  Deszendenziheorie,  1901,  chapter  i.) 
If  birds  and  mammals  are  two  branches  of  a  common  stock,  which  is  very 
doubtful,  they  are  still  not  directly  related,  but  are  only  connected  through 
long  extinct  Saurians.  The  question  whether  the  sparrow  and  the  crocodile 
have  branched  off  from  the  same  primitive  form  no  more  admits  of  a 
philosophical  answer,  than  does  the  question  regarding  the  sparrow  and  the 


soul ;  for  God's  almighty  power  cannot  have  produced  the 
soul,  which  is  a  spirit,  out  of  matter,  as  it  produced  the  forms 
of  plants  and  animals.1 

No  evolution  theory  is  capable  of  bridging  the  gulf  between 
the  mind  of  man  and  matter,  which  our  experience  teaches 
us  really  exists.  It  is  far  greater  than  the  gulf  between  in- 
organic matter  and  organised  substances,  or  than  that  between 
vegetative  and  sensitive  life  ;  its  width  is  such  that  it  will 
never  be  bridged,  just  because  mind  and  matter  are  diametri- 
cally opposed.  Modern  monism  has,  of  course,  forgotten 
this  ancient  truth,  and  is  doing  its  best  to  ignore  the  essential 
difference  between  them,  but  it  is  successful  neither  in  the 
mental  physiology  of  man,  nor  in  the  comparative  psychology 
of  man  and  beast ;  between  the  movement  of  the  atoms 
in  the  brain  and  thought,  between  animal  instinct  and  human 
intelligence,  yawns  ever  the  old  impassable  gulf.2 

Materialists  are  only  wasting  their  time  when  they  collect 
stone  after  stone  and  fling  them  into  the  abyss  ;  the  stones 
vanish  like  dust  in  a  bottomless  pit,  and  the  gulf  remains 
as  wide  as  ever.  Equally  futile  are  all  attempts  to  bridge 
it  by  references  to  the  '  intelligence  '  of  apes,  or  ants,  or  any 
other  animal,  and  by  depreciating  to  the  utmost  extent  the 

1  In  the  Biolpgisches  Zentralblatt  for  1903,  p.  602,    note  1,  Professor  L. 
Plate  expresses  his  disapproval  of  this  sentence,  and  criticises  it  as  '  curious  logic,' 
adding  :    '  That  God  is  almighty,  and  nevertheless  is  limited  in  His  sphere  of 
action,  is  a  contradictio  in  adiecto.'     Has  my  good  colleague  never  heard  that 
there  are  things  which  are  not  affected  by  God's  omnipotence,  because  they 
contain  an  interior  contradiction  ?   Does  he  perchance  fancy  that  God's  omnipo- 
tence could  make  2x2  =  5  and  not  =  4  ?      If  so,  he  has  a  more  exalted  idea 
of  the  divine  omnipotence  than  all  the  theologians  in  the  world  put  together. 
— Have,  pia  anima  ! 

2  A  classical  attempt  to  bridge  this  gulf  was  made  by  H.  E.  Ziegler  in  the 
lecture  already  mentioned, '  Tiber  den  derzeitigen  Stand  der  Deszendenzlehre  in 
der  Zoologie.'     On  p.  28  he  says  :    '  If  the  stag  can  be  related  to  the  roebuck, 
in  spite  of  the  fact  that  the  stag  has  large  antlers  and  the  roebuck  only  small 
horns,  so  man  can  be  related  to  beasts,  although  man  has  a  great  intellect 
and  beasts  only  a   small  one.'      This  profound   statement   deserves   to   be 
inscribed  in  golden  letters  on  the  annals  of  comparative  psychology,  that 
generations  to  come  may  benefit  by  it.     One  might  almost  fancy  that  it 
was  written  at  the  time  of  shedding  horns,  when  the  old  antlers  of  intellect 
had  been  cast,  and  the  new  ones  had  not  yet  grown.     Not  long  ago  H.  E. 
Ziegler  published  a  new  treatise  '  Uber  den  Begriff  des  Instinktes '  (Zoolog. 
Jahrbiicher,  Supplementary  volume,  VII,   1904,  pp.  700-726),  the  historical 
part  of    which    abounds  in  superficialities   and    biased   misrepresentations. 
The  author  unfortunately  gives  a  very  poor  account  of  instinct  as  it  is  usually 
understood  in  Christian  philosophy.     It  may  be  interesting  to  compare  the 
definition  of  instinct  given  in  my  book,  Instinkt  und  Intelligenz  im  Tierreich, 
1905,  pp.  23,  &c. 


mental  level  of  the  wildest  savages  ;  no  success  ever  has 
followed,  or  ever  will  follow,  such  attempts.  The  essential 
difference  between  the  mental  life  of  man  and  the  sentient 
existence  of  beasts,  and  the  impossibility  that  an  alleged 
brute  ancestor  of  man  should  ever  have  become  the  first 
homo  sapiens  by  natural  evolution,  are  facts  that  cannot  be 
set  aside.1  Therefore,  it  is  a  real  *  postulate  of  sciencej  to 
account  for  the  mind  of  man  by  an  act  of  creation.  This 
involves  no  violation  of  the  laws  of  nature  ;  for"  as  mind 
cannot  be  produced  out  of  matter,  it  is  obvious  that  origin  by 
creation  is,  in  the  case  of  mind,  the  only  natural  mode  of  origin. 

2.  We  have  now  completed  our  examination  of  the 
philosophical  limits  to  the  theory  of  evolution  and  may 
pass  on  to  those  assigned  to  it  by  natural  science,  although 
here,  too,  we  must  begin  with  a  philosophical  preamble. 

The  theory  of  evolution  is  a  scientific  hypothesis,  and  in 
its  further  development  is  a  scientific  theory.  By  an  hypo- 
thesis is  meant  a  proposition,  the  truth  of  which  cannot  be 
demonstrated  directly  by  way  of  observation  or  experiment, 
but  which  follows  as  a  reasonable  deduction  from  facts,  because 
it  is  capable  of  supplying  a  satisfactory  explanation  of  them. 
Hypotheses  or  suppositions  are  indispensable  in  natural 
science  ;  without  them  there  is  in  fact  no  science  in  this  depart- 
ment of  knowledge,  for  science  is  scientia  rerum  ex  causis  ; 
so,  apart  from  hypotheses,  we  should  have  only  a  crass 
empiricism,  contenting  itself  with  observations,  and  caring 
nothing  for  the  why  and  the  wherefore  of  them.  As  our 
immediate  perception  of  the  things  of  nature  around  us  reveals 
to  us  only  their  outer  husk,  our  mind  is  forced  to  have  recourse 
to  hypotheses,  in  order  at  least  to  some  extent  to  be  able  to 
penetrate  into  the  working  of  the  laws  of  nature.  If  various 
modes  present  themselves  of  explaining  one  and  the  same 
phenomenon  or  group  of  phenomena,  the  mind  compares 
and  examines  them  to  see  which  agrees  best  with  the  facts 
that  bear  upon  the  subject,  taken  collectively.  One  is  then 
selected  as  the  most  probable  hypothesis,  which  the  student 
of  nature  must  accept,  until  a  better  is  found. 

1  On  this  subject  cf.  my  two  works,  Instinkt  und  Intdligenz  im  Tierreich, 
1905,  and  Vergleichende  Studien  uber  das  Seelenleben  der  Ameisen  und  der 
hoheren  Tiere,  1900. 


As  an  hypothesis  obtains  additional  probability  when 
pieces  of  evidence  from  various  sources  concur  to  establish 
it,  it  develops  into  a  uniform  scientific  structure,  and  ceases 
to  be  an  hypothesis  and  becomes  a  theory.  The  nature  of 
things  requires  that  we  can  never  Jemalid  such  a  degree 
of  certainty  for  a  scientific  hypothesis,  or  even  for  a  theory, 
as  for  a  mathematical  formula.  Metaphysical  (mathematical) 
certainty  can  never  exist  with  regard  to  it,  and  physical 
certainty  only  seldom  ;  as  a  rule  it  can  only  claim  a  lower  or 
higher  degree  of  probability.  The  Copernican  theory  supplies 
us  with  an  instance  how  an  hypothesis,  originally  possessing 
only  a  moderate  degree  of  probability,  may  eventually  rise 
to  the  rank  of  a  theory,  having  so  much  physical  certainty 
that  at  the  present  day  no  educated  person  doubts  its  accuracy. 
It  would  be  unfair  to  demand  at  the  outset,  in  order  to  justify 
the  scientific  existence  of  an  hypothesis,  that  irrefutable 
evidence  in  support  of  it  should  be  adduced.  To  demand 
this  would  be  almost  as  foolish  as,  before  partaking  of  any 
food,  to  require  a  chemical  guarantee  that  it  contains  no  poison. 

Let  us  now  apply  these  principles  to  the  theory  of  evolution. 
The  weight  of  the  evidence  in  its  favour  is  as  often  diminished 
by  exaggeration  of  its  value  on  the  part  of  its  champions, 
as  by  depreciation  of  its  cumulative  force  on  the  part  of  its 

With  regard  to  the  nature  and  origin  of  the  organic  species, 
we  have  to  choose  between  two  opposite  theories,  each  of 
which  consists  of  a  group  of  connected  hypotheses.  Of  these 
theories  one,  that  of  permanence,  maintains  the  absolute 
invariability  of  the  systematic  species.  It  is  of  opinion  that 
the  species  are  perfectly  unchangeable,  although  varieties  and 
breeds  may  be  formed  within  them ;  therefore  it  regards 
relationship  between  the  species  as  impossible,  and  as  equally 
impossible  the  suggestion  that  our  present  species  can  be  the 
descendants  of  other  extinct  ones.  Consequently  it  assumes 
so  many  special  acts  of  creation  to  have  been  performed  as 
there  are  distinct  systematic  species,  and  we  may  assume 
that  at  least  800,000  are  known  to  exist  now.  But  in  the 
various  geological  periods,  as  a  rule,  species  have  followed 
one  another, — they  appear  at  the  beginning  of  a  period  and 
vanish  at  its  close  ;  so  that  this  theory  requires  the  acts  of 


creation  to  have  been  constantly  repeated  during  the  whole 
geological  evolution  of  our  earth.  '  But  why,'  some  one  may 
ask,  'need  we  lay  so  extreme  an  interpretation  upon  the 
theory  of  permanence  ?  Why  do  we  not  rather  say  that  it 
requires  a  relative,  but  not  an  absolute,  invariability  of  the 
species  ?  '  Simply  because  to  accept  a  merely  relative  per- 
manence of  the  species  involves  necessarily  the  acceptance 
of  a  relative  variability.  A  theory  of  permanence,  which 
declares  the  systematic  species  to  be  '  relatively  variable/ 
regards  them  as  variable  either  only  within  the  limits  of  the 
species  or  beyond  those  limits.  In  the  first  case  it  asserts 
practically  the  absolute  permanence  of  the  limits  of  the 
species,  and  restricts  the  variability  to  the  characteristic 
marks  of  the  varieties  and  breeds  within  the  species  ;  in  the 
second  case,  on  the  contrary,  it  ceases  to  be  a  theory  of  per- 
manence, for  it  accepts  the  principle  of  the  theory  of  evolution, 
which  regards  the  systematic  species  as  related  by  belonging 
to  a  common  stock.  It  must  not  be  forgotten  that  the  historic 
strife  between  the  theories  of  permanence  and  descent  concerns 
the  systematic  species  in  natural  science,  not  the  so-called 
natural  species.  Our  idea  of  the  latter  is  based  on  natural 
philosophy,  and  has  taken  its  present  form  under  the  influence 
of  the  theory  of  evolution.  I  shall  have  to  recur  to  it  in  the 
next  section  of  this  chapter. 

Our  second  alternative  is  the  theory  of  evolution,  according 
to  which  the  organic  species  have  been  evolved  from  earlier 
forms  belonging  to  previous  ages.  It  holds  that  the  species 
are  relatively  permanent  for  a  definite  geological  period, 
and  that  palseontological  research  shows  shorter  periods  of 
transformation  to  alternate  with  longer  periods  in  which  the 
organic  forms  do  not  vary.1  We  are  now  in  one  of  the  latter, 
more  permanent  periods,  and  this  explains  the  normal  per- 
sistence of  our  systematic  species  ;  they  correspond  to  the 
conditions  of  life  around  them ;  but  as  there  is  only  a  relative, 
and  not  a  fundamental  difference  between  the  characteristics  of 

1  Cf.  Zittel,  Grundzuge  der  Palaontologie,  1903,  p.  15.  Attention  was  drawn 
to  this  phenomenon  by  Oswald  Heer  in  his  Urwelt  der  Schweiz,  1883,  chapter 
xviii.  What  de  Vries  calls  the  '  periods  of  mutation,'  and  the  periods  of 
*  explosive '  transformation  of  species  (Koken,  Standfuss),  are  only  other 
names  for  the  above-mentioned  periods  of  change.  The  view  which  de  Vries 
takes  of  his  '  periods  of  mutation  '  is  extremely  hypothetical  (Mutations- 
theorie,  II,  1903,  §  12,  p.  697). 


species  and  of  genera  in  systematics,  this  theory  extends  the 
idea  of  a  natural  evolution  also  to  the  origin  of  genera.  The 
genera  of  systematic  classification  are  only  groups  of  natural 
species,  more  closely  akin  to  one  another  than  to  the  species  of 
other  groups,  although  they  may  originally  have  branched  off 
from  the  same  stock.  The  theory  of  evolution  affects  families 
and  orders  in  the  same  way,  and,  as  far  as  facts  allow,  also  the 
higher  divisions  of  the  animal  and  vegetable  kingdoms.  So 
much  for  the  theory. 

What  are  the  limits  of  the  theory  from  the  point  of  view 
of  natural  science  ?  How  far  do  facts  enable  it  to  answer  the 
three  following  questions,  with  which  philosophy  cannot  deal  ? 
At  what  date  did  organic  life  begin?  Must  we  assume  the 
evolution  of  plants  and  animals  to  have  been  monophyletic  or 
polyphyletic  ?  What  internal  and  external  causes  gave  rise 
to  the  hypothetical  race-evolution  ? 

We  know  very  little  as  yet  regarding  the  date  when  living 
organisms  first  appeared  upon  our  earth.  It  is  certain  that 
life  was  possible  only  after  the  surface  of  the  earth  had 
cooled  down,  and  had  formed  an  atmosphere  about  itself.  The 
earliest  organisms  probably  lived  in  the  water.1  In  geological 
language,  the  date  of  the  first  appearance  of  organic  life 
coincides  with  the  end  of  the  Azoic  and  the  beginning  of  the 
Palaeozoic  age.  The  dividing  line  between  these  two  periods 
in  the  history  of  our  planet  must  probably  be  set  further  back 
than  has  hitherto  been  done.  It  is  well  known  that  geologists 
used  to  regard  the  Cambrian  formation  as  the  oldest  stratum 
containing  fossils.  But  recently  Pre- Cambrian  fossils  have 
been  found  in  North  America,  Great  Britain,  Scandinavia, 
Bohemia,  and  elsewhere,  so  that  now  the  Pre-Cambrian  is 
regarded  as  the  oldest  stratum  containing  fossil  remains  of 
living  creatures.2  In  the  present  state  of  our  knowledge  it  is 
still  quite  impossible  for  us  to  fix  the  age  of  this  stratum  ;  very 
likely  millions  of  years  have  passed  between  the  time  when 
it  was  formed  and  now. 

1  Dependent  on  this  is  the  further  question  whether  the  first  centres  of 
creation  were  at  the  poles,  i.e.  at  the  ends  of  the  shortest  axis  of  the  earth, 
or  in  the  equatorial  zone,  at  the  ends  of  the   longest  axis.      On  the  latter 
hypothesis  see  Simroth,  '  Uber  das  natiirliche   Sj'stem  der  Erde  '  ( Verhandl. 
der  Deutschen  Zoolog.  Gesellschaft,  1902,  pp.  19-42). 

2  Cf.  on  this  subject,  Credner,  Elemente  der  Geologie,  1902,  pp.  389-394 ; 
R.  Hcrtwig,  Lehrbuch  der  Zoologie,  p.  151  (English  translation,  p.  180). 


We  do  not  know  whether  the  primitive  forms  of  all  the 
creatures  that  lived  later,  of  all  classes  in  the  animal  and 
vegetable  kingdoms,  existed  in  the  Pre-Cambrian  period.  Prob- 
ably they  did  not,  for,  as  far  as  we  know,  vertebrates  appeared 
first  in  the  Silurian,  and  flowering  plants  seem  to  be  of  still 
later  origin.  Whether  the  occurrence  of  any  particular  class 
of  forms  was  really  the  first  or  not,  is  a  point  on  which  no  final 
answer  can  be  given,  and  therefore,  from  the  scientific  stand- 
point, we  are  still  far  from  being  able  to  decide  whether  the 
primitive  types  of  the  chief  classes  of  animals  and  plants  were 
produced  simultaneously  or  in  succession,  nor  can  we  say 
when  they  first  appeared. 

I  may  here  give  a  short  sketch  of  what  palaeontology 
teaches  us  regarding  the  sequence  of  plant  and  animal  forms  in 
the  course  of  the  earth's  history.  The  list  of  the  geological 
strata  with  the  names  of  the  various  formations  has  been 
already  given  (p.  253),  and  I  need  not  repeat  it  here. 

In  speaking  of  animals  I  shall  follow  chiefly  Zittel's  '  Grund- 
ziige  der  Palaontologie,'  and  E.  Hertwig's  'Lehrbuch  der 
Zoologie.'  No  living  organisms  can  be  assigned  with  certainty 
to  the  Azoic  or  archaic  age.  The  animal  nature  of  the  famous 
eozoon  found  in  the  Archaean  (Laurentian)  strata  is,  to  say  the 
least,  very  doubtful.  The  Palaeozoic  age  supplies  the  earliest 
organisms.  In  the  Pre-Cambrian  strata  of  Brittany  there  are 
numerous  remains  of  Radiolaria,  if  Barrois  is  correct  in  his 
interpretation  of  the  discoveries  made.  The  Cambrian  strata 
contain  only  remains  of  various  classes  of  invertebrates, 
amongst  which  Arthropods  (Trilobites),  Brachiopods  (reckoned 
by  Hertwig  among  Worms),  Echinoderrns  and  Molluscs  are 
the  chief.  In  the  Silurian,  besides  the  above-mentioned,  occur 
the  first  vertebrates  of  the  class  of  fishes,  and  the  first  insects 
among  the  Arthropods.  In  the  Devonian  there  are  many 
different  kinds  of  fishes.  In  the  Carboniferous  begin  the 
Amphibia,  and  in  the  Permian  the  reptiles.  In  many  cases 
the  forms  of  these  palaeozoic  creatures  very  closely  resemble 
those  of  the  modern  representatives  of  the  same  classes  (Nauti* 
lus,  Lingula) ,but  as  a  rule  they  are  very  different  (e.g.  Trilobites), 
although  frequently  they  are  not  inferior  to  their  modern 
relations  in  their  degree  of  organisation.  The  Mesozoic  age 
is  that  in  which  reptiles  reached  their  highest  development, 


and  the  insect  fauna  of  the  Lower  Jurassic  or  Lias  is  very 
numerous.  The  first  mammals  appear  in  the  Triassic  or 
earliest  Mesozoic  age,  and  in  the  Upper  Jurassic  the  first  birds, 
if  we  may  reckon  the  Archaeopteryx  as  a  genuine  bird,  in  spite 
of  its  many  points  of  resemblance  to  a  reptile.  The  fauna  of 
the  Csenozoic  age  approaches  more  and  more  to  that  of  the 
present  time  ;  in  the  Tertiary  period  the  still  existent  orders 
of  mammals  and  birds  developed,  and  the  likeness  between  the 
insects  of  that  period  and  our  own  is  still  more  striking.  Man 
appeared  only  in  the  Quaternary  period,  on  the  threshold  of 
modern  times. 

According  to  Eeinke's  '  Philosophie  der  Botanik '  (pp.  132, 
&c.)  the  geological  sequence  of  plant-forms  is  as  follows. 
There  are  no  remains  at  all  of  plants  in  the  Pre-Cambrian  and 
Cambrian  strata  ;  the  earliest  are  ferns,  which  occur  in  the 
Silurian,  at  the  same  time  as  the  first  land  animals  (insects).  Of 
other  Cryptogams,  the  chalk-algae  also  occur  in  the  Silurian,  the 
flint-algae  in  the  Carboniferous  strata,  and  they  form  enormous 
deposits  in  the  Chalk  and  Tertiary  strata.  Ferns,  shave-grasses, 
and  Lycopodia  reached  the  highest  point  of  their  development 
in  the  Coal  age,  and  had  then  in  some  ways  a  more  perfect 
organisation  than  at  the  present  time.  There  are  no  fossils 
that  can  serve  as  links  connecting  the  Algae  and  the  mosses, 
or  the  mosses  and  the  ferns. 

The  Gymnosperms  were  the  first  Phanerogams  to  make 
their  appearance.  The  earliest  of  them  are  the  Cordaitae, 
relations  of  the  Cycadaceae,  which  appear  first  in  the  Devonian, 
reach  their  highest  point  in  the  Carboniferous,  and  vanish  in 
the  Permian.  The  first  undoubted  remains  of  Cycadaceae 
occur  in  the  Permian,  as  well  as  the  first  Ginkgos  and  Conifers. 
In  the  Mesozoic  age,  in  the  Triassic,  Jurassic  and  Cretaceous 
periods,  the  three  above-mentioned  families  of  Gymnosperms 
developed  still  further,  and  in  the  Tertiary  strata  occur  only 
such  kinds  as  are  still  known.  The  earliest  Angiosperms,  both 
monocotyledons  and  dicotyledons,  appear  suddenly  in  a  great 
variety  of  forms  in  the  Upper  Chalk,  and  are  unconnected 
with  the  Gymnosperms  that  preceded  them.  During  the 
Tertiary  period  more  and  more  representatives  occur  of  still 
existent  families,  genera  and  species  of  Gymnosperms,  and 
their  frequency  increases  in  the  more  recent  strata. 


What  information  as  to  the  hypothetical  history  of  the 
primitive  forms  in  the  organic  world  is  given  us  by  palaeon- 
tology in  its  two  branches,  palseozoology  and  palaeophytology  ? 
It  tells  us  nothing  certain  as  to  the  date  of  the  appearance 
of  the  first  living  organisms  or  as  to  their  structure,  for  those 
organisms  alone  could  be  preserved  as  fossils  which  were 
solid  enough  to  make  impressions  or  hollows  in  the  stone  ; 
all  soft  protoplasmic  formations  must  have  perished  and 
left  no  trace.  Moreover,  it  gives  us  only  faint  suggestions, 
though  they  are  extremely  valuable,  as  to  the  order  in  which 
the  chief  classes  of  animals  and  plants  appeared  upon  earth, 
but  it  affords  certain  evidence  that  the  Fauna  and  Flora  of 
former  ages  gradually  approximated  more  and  more  to  those 
of  the  present  time.  Numberless  families  and  genera  of 
ancient  animals  and  plants  have  become  extinct,  some  long 
ago,  some  more  lately,  leaving  no  descendants  ;  but  on  the 
other  hand  very  many  seem  to  have  been  really  the  ancestors 
of  our  present  Fauna  and  Flora,  in  spite  of  the  inevitable  gaps 
in  the  palaeontological  records,  and  in  spite  of  the  uncertainty 
still  attaching  to  the  interpretation  to  be  put  upon  many 
palaeontological  discoveries.1 

Let  us  now  turn  to  the  second  question  and  ask  :  *  Are 
we  to  assume  that  the  evolution  of  animals  and  plants  was 
monophyletic  or  polyphyletic  ?  '  There  is  no  trace  of  any 
scientific  evidence  to  show  that  the  two  organic  kingdoms 
were  descended  from  one  common  primitive  cell.  It  is  true 
that  now  every  multicellular  organism  in  its  ontogeny  proceeds 
from  a  unicellular  stage,  and  among  unicellular  organisms 
there  are  many  of  which  it  is  impossible  to  decide  whether 
they  are  plants  or  animals  ;  but  it  is  a  very  bold  speculation 
to  conclude  from  these  considerations  that  all  organisms  are 
descended  from  a  common  ancestral  cell.  We  are  quite  ignorant 
too  as  to  whether  we  must  assume  the  vegetable  kingdom  and 
the  animal  kingdom  respectively  to  have  had  a  monophyletic 
or  polyphyletic  evolution.  This  alone  is  certain  ;  there  is 
no  evidence  at  all  in  support  of  a  monophyletic  phylogeny. 
"  All  honest  supporters  of  the  theory  of  evolution,  who 

1  Tn  his  book  on  the  theory  of  descent  (Die  Deszendenztheorie)  Fleischmann 
has  emphasised  these  two  points  as  detrimental  to  the  theory  of  evolution, 
but  he  has  exaggerated  their  importance.  Cf.  the  discussion  in  Stimmen  aus 
Maria-Laach,  LXII,  1902,  Part  I,  pp.  116,  &c. 

u  2 


pay  due  attention  to  facts,  acknowledge  further  that  the 
grounds  for  assuming  the  existence  of  a  real  relationship 
between  the  forms  in  question  become  more  scanty  when 
the  higher  divisions  of  the  system  are  considered.  For  the 
species  of  one  genus  these  grounds  often  amount  to  great 
and  even  irrefutable  probability,1  and  the  same  may  be  said 
in  not  a  few  cases  of  the  genera  of  one  family,  and  occasionally 
for  the  families  of  one  order,  but  it  can  seldom  be  maintained 
of  the  orders  of  one  class.  The  evidence  afforded  by  natural 
science  for  the  theory  of  common  descent  becomes  steadily 
weaker  the  higher  we  ascend  in  the  system,  and  it  becomes 
weaker,  too,  the  deeper  we  go  into  the  palseontological  history 
of  our  earth  in  order  to  seek  the  common  ancestors  of  the 
subsequently  distinct,  systematic  divisions. 

In  the  latest  (7th)  edition  (1905,  p.  152)  of  his  '  Lehrbuch 
der   Zoologie '    K.   Hertwig   gives   the   chief   natural   groups 
of  the  animal  kingdom  as  seven  in  number  (Protozoa,  Coelen- 
terata,  Worms,  Echinodermata,  Mollusca,  Arthropoda,  Verte- 
brata)  ;    C.  Glaus   reckons  nine,  and  the  number  is  variously 
given  by  other  zoologists ;  but  the  evidence  in  support  of 
the  theory  that  these  groups  are  of  common  origin  is  so  weak 
that  we  must  describe  it  as  improbable  rather  than  probable, 
in  the  present  state  of  our  knowledge.     The  truth  of  this 
statement    becomes    apparent    if    the    different    hypotheses 
be  compared  ;     for  instance,  those  put  forward  to  account 
for  the   descent  of  Vertebrata  or  of  Arthropoda  from  other 
groups  of  animals  ;   with  regard  to  these  hypotheses  we  might 
almost  say  :    Quot  capita,  tot  sensus.     When  the  opinions  of 
scientists  diverge  so  greatly  on  one  and  the  same  point,  we  may 
safely  conclude  that  nothing  certain  is  known  about  it.  Whether 
we  accept  seven  or  seventeen,  or  any  other  number,  as  that  of 
the  chief  types  of  the  animal  kingdom,  it  is  always  impossible 
to  assign  to  them  a  monophyletic  descent  from  a  common 
primitive  form.     This  has  been  thoroughly  proved  by  Hamann 
('  Entwicklungslehre  und  Darwinismus,'  1892),  and  by  Pleisch- 
mann  (*  Die  Deszendenztheorie,'  1901)  ;  recently  even  Theodor 
Boveri  expressed  the  same  opinion  in  his  rectorial  address 
on  May  11,   1906   ('  Die  Organismen  als  historische  Wesen,' 
Wiirzburg,  1906,  pp.  7  and  51). 

1  Instances  of  this  will  be  given  in  Chapter  X.     See  also  pp.  276,  &c. 


The  same  holds  good  with  regard  to  the  chief  classes 
among  plants  ;  R.  von  Wettstein  thinks  that  we  must  dis- 
tinguish seven,  all  independent  of  one  another  ('  Handbuch 
der  systematischen  Botanik,'  I,  1901,  p.  16). 

In  fact,  among  modern  zoologists  and  botanists,  and  still 
more  among  palaeontologists,1  the  number  is  ever  increasing 
of  those  who  think  that  the  evolution  of  both  animals  and 
plants  was  polyphyletJCj  and  who  regard  the  monophyletic 
hypothesis  as  merely  a  pretty  fancy  on  the  part  of  the  supporters 
of  the  theory  of  descent  in  its  crude  form — a  fancy  that  they 
cannot  hope  to  prove  true,  for  comparative  morphology  and 
ontogeny  of  living  organisms,  as  well  as  the  discoveries  made 
by  paleontology,  all  alike  render  it  more  and  more  improbable 
that  anyone  will  ever  succeed  in  establishing  a  monophyletic 
evolution  of  either  the  animal  or  the  vegetable  kingdom  on  a 
scientific  basis.  It  becomes  more  and  more  probable  that 
anionophyletic  evolution  does  not  correspond  at  all  with 

"~No  serious  student  is  at  present  able  to  tell  us  with  cer- 
tainty how  many  independent  lines  of  descent,  or  series  of 
evolution,  we  must  assume  to  exist  among  animals  and  plants 
respectively.  This  is  due  partly  to  the  fact  that  the  answer 
to  this  question  depends  greatly  upon  the  subjective  ideas 
of  each  individual,  but  the  chief  reason  for  it  lies  in  the  signi- 
ficant circumstance  that  a  final  answer  will  be  possible  only 
when  we  have  a  perfect  knowledge  of  both  the  present  and 
the  fossil  organic  world.  At  the  present  day  we  are  at  an 
immense  distance  from  possessing  such  knowledge,  and  there- 
fore we  do  not  know  how  many  original  acts  of  creation  must 
be  assumed,  in  order  to  account  for  the  existence  of  the  living 
organisms  in  the  world.  Koken  says  on  this  subject  (1902, 
p.  218)  :  '  All  the  great  Phyla  go  back,  sharply  distinguished, 
to  the  Cambrian  period,  and  we  have  no  records  at  all  of 
those  periods  when  they  might  have  been  connected,  or  when 
they  branched  off  from  a  common  stock.'  Steinmann  (1899, 

1  Cf.  on  this  subject. E.  Koken,  Die  Vorwelt  und  ihre  Entwicklungsgeschickte, 
Leipzig,  1893  ;  '  Palaontologie  und  Deszendenzlehre,'  address  given  at  the 
seventy-third  meeting  of  German  naturalists  at  Hamburg,  on  September  26, 
1901  (Verhandl.  I,  Leipzig,  1902,  pp.  212-228.  Reprinted  Jena,  1902).  G. 
Steinmann,  Die  Erdgeschichtsforschung  wdhrend  der  letzten  vier  Jahrzehnte 
(Freiburg  i.  B.,  1899);  Palaontologie  und  Abstammungslehre  am  Ende  des 
Jahrhunderts  (ibid.,  1899). 


p.  33)  goes  so  far  as  to  believe  that  men  will  never  attain  to 
this  knowledge  :  '  I  feel  certain  that  the  oldest  representatives 
of  animals  and  plants  of  every  kind  will  for  ever  remain  un- 
known to  us  ;  all  trace  of  them  has  probably  vanished,  owing 
to  the  great  changes  undergone  by  the  oldest  strata.' 

We  still  do  not  know,  and  probably  we  shall  never  know, 
under  what  form  we  are  to  imagine  the  hypothetical  primitive 
types  of  the  various  series  of  evolution  ;  whether  we  are  to 
think  of  them  as  very  simple  cells,  having  however  an  already 
definite  tendency  or  Anlage  to  evolution  ;  or  as  phylembryos, 
or  as  further  differentiated  forms,  displaying  the  exterior 
characteristics  of  the  various  types  in  the  shape  of  definite 
morphological  designs.  Nor  can  we  state  anything  as  to  the 
appearance  of  these  primitive  types  ;  we  do  not  know  whether 
they  all  appeared  at  the  same  time,  or  in  succession,  nor  when 
they  were  produced. 

We  come  now  to  the  third  question  :  *  What  does  natural 
science  tell  us  of  the  interior  and  exterior  causes  of  the 
hypothetical  race-evolution  ?  '  Here  we  are  still  more  com- 
pletely in  the  dark.  Leaving  aside  those  prejudiced  persons 
who  are  blindly  in  love  with  their  own  theory — the  theory 
of  selection,  or  orthogenesis,  or  whatever  it  is — and  fancy 
that  it  explains  everything  (although,  as  a  matter  of  fact,  it 
explains  very  little),  we  may  frankly  acknowledge  that  our 
knowledge  of  the  reaL  causesof  the_^ac^e-evolution_Qf  the 
organic  species  is  still  in  itslrjiaiic£^0ne  thing  alone  seems  to 
be  fairly^  certain  :  Numerous  interior  and  exterior  factors 
must  be  regarded  as  the  causes  of  the  race-evolution,  and 
the  part  played  by  these  factors  with  respect  to  various  series 
in  evolution  differs  greatly  as  to  the  extent  both  of  their 
participation  and  co-operation.1 

Just  as,  in  the  development  of  the  individual  organism,  pre- 
formation  and  epigenesis  work  together  in  accord,2  and  definite 
interior  tendencies  are  regularly  modified  by  exterior  influ- 
ences, so,  as  we  may  suppose,  is  it  in  the  race-evolution  of 
living  organisms.  In  general  we  must  follow  Nageli  in  dis- 
tinguishing, in  the  case  of  organic  species,  characteristics  due 

1  Some  instances  taken  from  zoology  will  be  found  in  Chapter  X. 

2  See   Chapter  VIII,  p.   225,  and   p.  235.     Also   0.   Hertwig,   Allgemeine 
Biologie,  1906,  pp.  132,  &c.,  pp.  138,  &c. 


to  organisation  from  those  due  to  adaptation.  The  former, 
which  determine  the  degree  of  organisation,  must  primarily 
be  referred  to  the  interior  causes  of  evolution,  whilst  the 
latter  are  connected  with  the  influence  of  the,  exterior  causes. 
The  active  parts  taken  by  both  series  of  causes  are  more  or 
less  mixed,  and  the  interior  causes  are  always  the  foundation, 
acted  upon  by  the  exterior  (e.g.  nutrition,  temperature,  light, 
&c.),  which  affect  evolution  by  means  of  various  attendant 

I  cannot  at  present  discuss  this  topic  further.  I  have 
considered  both  the  philosophical  and  the  scientific  limitations 
of  the  theory  of  evolution,  and,  as  I  believe,  have  dealt 
impartially  with  both  philosophy  and  science.  We  must 
not  undervalue,  but  neither  must  we  overvalue,  the  achieve- 
ments of  the  theory  of  evolution  hitherto.  Centuries 
will  pass  before  it  succeeds  in  establishing,  with  a  sufficient 
degree  of  probability,  the  number  of  primitive  series  of  animals 
and  of  plants  respectively,  and  in  arranging  correctly  the 
forms  belonging  to  each  series  in  the  many  ramifications  of 
their  relationship.  Centuries  more  must  elapse  before  science 
will  be  able  to  trace  back  these  series  to  their  origin,  and  to 
discover  the  primitive  forms  of  each.  And  centuries  of  research 
will  be  required  before  men  will  find  a  satisfactory  explana- 
tion of  the  causes  which  control  evolution  within  each  series 
of  forms.  Shall  we  therefore  be  contented  to  say  :  *  Before 
we  acknowledge  the  theory  of  evolution  to  have  a  scientific 
justification,  we  had  better  wait  until  it  has  accomplished 
all  these  tasks  ?  '  To  do  so  would  be  both  unreasonable 
and  foolish.  On  the  contrary,  we  can  only  wish  that  as  many 
serious  research-students  as  possible  may  apply  themselves 
with  all  zeal  to  solving  the  difficult  problems  connected  with 
the  theory.  This  solution  could  not  fail  to  benefit  philosophy, 
whilst  it  would  be  far  more  creditable  to  the  theory  of  evolution 
for  its  supporters  to  proceed  thus,  than  to  act  like  Haeckel  and 
those  who  share  his  opinions,  and  try  to  popularise  the  theory 

1  Cf.  also  p.  176  and  p.  282  ;  also  B.  von  Wettstein,  Berichte  der  botanischen 
Gesellschaft,  XVIII,  1900,  pp.  184-200 ;  E.  Koken,  Paldontologie  und  Deszendenz- 
khre,  1902  ;  Ed.  Fischer,  '  Die  biologischen  Arten  der  parasitischen  Pilze 
und  die  Entstehung  neuer  Formen  im  Pflanzenreich '  ( Verhandl.  der  Schweizer 
Naturforschergesellschaft,  eighty-sixth  annual  meeting,  Locarno,  September 
1903) ;  Uber  den  heutigen  Stand  der  Deszendenzlehre  und  unsere  Stellung  zu 
derselben,  Berne,  1904. 


to  advance  their  own  ends,  and  make  a  wrong  use  of  it  as 
a  weapon  with  which  to  attack  the  Christian  cosmogony. 


Linnaeus,  who  is  to  be  regarded  as  the  originator  of  our 
present  conception  of  systematic  species,  and  who,  therefore,  has 
been  called  the  father  of  the  theory  of  permanence,  enunciated 
the  following  dictum  :  Tot  species  numeramus,  quot  diversae 
formae  in  principio  sunt  creatae — we  reckon  so  many  (syste- 
matic) species  as  there  were  different  forms  created  in  the 

How  must  this  dictum  be  worded  to  make  it  agree  with  the 
theory  of  evolution  ?  According  to  it,  the  systematic  species 
of  the  present  time  do  not  represent  the  originally  created  forms, 
but  are  the  result  of  a  process  of  evolution,  uniting  the  species 
of  the  present  and  the  past  in  natural  series  of  forms,  the 
members  of  which  are  related  to  one  another,  and  each  of 
which  points  back  to  an  original  primitive  form,  whence  it 
is  derived.  If  we  designate  each  of  these  independent  series 
of  forms,  not  related  to  other  series  or  families,  as  a  natural 
species,1  we  can  still  assent  to  Linnaeus's  dictum  :  Tot  species 
numeramus,  quot  diversae  formae  in  principio  sunt  creatae.  We 
reckon  so  many  natural  species  as  there  were  different  primitive 
forms  created  in  the  beginning.2  Each  of  these  natural  species 

1  A  similar  view  regarding  natural  species   has  already  been  expressed 
by  Father  T.  Pesch  in  his  Philosophia  naturalist,  II,  p.  334,  in  order  to  explain 
the  facts  supporting  the  theory  of  evolution.     He  quotes  a  number  of  passages 
from  St.  Thomas  Aquinas  and  from  Suarez  in  favour  of  his  view.     Of  course 
we  are  here  speaking  of  the  species  physicae  of  natural  philosophy,  not  of 
the  species  metaphysicae  of  logic.     Almost  inconceivable  mistakes  as  to  my 
definition  of  natural  species  have  been  made  by  many  reviewers  of  the  first 
edition  of  this  work,  some  of  them  being  experienced  zoologists.     Escherich 
in  the  Supplement  to  the  Allgemeine  Zeitung  for  February  10,  and  11,  1905, 
gave  it  far  too  narrow  an  interpretation,  and  Haeckel,  Forel  and  others  simply 
followed    him  and  made  the  same  mistake,  without  examining  the  matter 
for  themselves.     Another  mistake  was  made  by  Friese  (Wiener  Entomologische 
Zeitung,  1904,  No.  10)  and  Schroeder  (Zeitschrift  fur  wissenschaftl  Insekten- 
biologie,  1905,  Part  4),  who  believe  my  distinction  between  systematic  and 
natural  species  to  be  identical  with  that  between  biological  and  morphological 
species  ;    the  biological  and  the  morphological  species  are  but  two  different 
aspects  of  the  systematic  species,  whilst  the  natural  species  comprises  all 
the  members  of  the  same  line  of  ancestry  or  pedigree,  and  therefore  is  much 
wider  from  the  point  of  view  of  natural  science.     I  trust  that  these  remarks 
will  prevent  further  misunderstandings. 

2  For  readers  who  have  studied  philosophy,  it  is  perhaps  needless  to  remark 
again  (as  I  do  for  the  benefit  of  some  of  my  critics),  that  the  creation  of  the 


has  in  the  course  of  evolution  differentiated  itself  into  more 
or  less  systematic  species.  How  many  systematic  species, 
genera,  and  families  belong  to  a  natural  species,  cannot  yet  be 
stated  with  certainty  in  most  cases.  Still  less  are  we  able 
to  say  how  many  natural  species  there  are,  i.e.  how  many  lines 
of  ancestry  independent  of  one  another.  We  must  leave  the 
decision  to  the  phylogenetic  research  of  future  ages,  if  indeed 
it  ever  succeeds  in  arriving  at  one. 

The  varying  degrees  of  capacity  for  evolution  possessed 
by  the  primitive  forms  of  the  different  natural  species  depend 
primarily  upon  the  interior  laws  of  evolution  impressed  upon 
their  organic  constitution ;  we  are  probably  justified  in 
regarding  the  chromatin  substance  of  the  germ-cells  as  the 
material  designed  to  transmit  these  laws.1  The  interaction  of 
these  interior  factors  in  evolution  and  of  the  surrounding 
exterior  influences,  through  which  many  kinds  of  adaptation 
came  about,  have  produced  the  ramifications  from  the  parent 
stock  of  the  natural  species,  and  they  have  been  affected  also 
by  cross-breeding  (amphimixis)  and  natural  selection. 

But,  it  may  be  asked,  what  is  the  practical  advantage  of 
distinguishing  thus  natural  and  systematic  species,  if  we  are 
still  unable  to  determine  which  forms  actually  constitute  a 
natural  species,  and  how  many  such  natural  species  there  are  ? 
To  this  question  we  may  answer  :  Firstly,  in  many  cases  we 
are  able  at  the  present  day  to  decide  in  some  degree  the  group 
of  forms  which  belong  to  a  natural  species,  although  we  may 
not  yet  know  with  certainty  its  full  extent*  For  instance  we 
may  reckon,  as  belonging  to  one  natural  species,  all  the  varieties 
of  beetle  of  the  Paussidae  family,  from  the  Tertiary  period 
to  the  present  time ; 3  but  as  the  Paussidae,  even  if  they  are 
the  outcome,  not  of  a  monophyletic,  but  of  a  diphyletic 
evolution  (cf.  Chapter  X,  §  9),  are  related  phylogenetically  to 

first  organisms  is  not  to  be  understood  as  a  creatio  e  nihilo,  but  as  a  production 
of  organisms  out  of  matter.  On  this  subject  see  the  sections  on  Spontaneous 
Generation  (p.  193),  and  on  the  Philosophical  Limitations  of  the  Theory  of 
Evolution  (p.  279). 

1  See  Chapter  VI,  p.  169  and  p.  177,  &c. 

2  I  have  italicised  these  words  because  they  were  overlooked  by  Escherich 
and  other  reviewers  in  the  former  edition. 

3  Cf.  Stimmen  aus  Maria-Laach,  LIU,  1897,  pp.  400  and  520,  &c.,   '  Die 
Familie  der  Paussiden' ;  also  'Neue  Beitrage  zur  Kenntnis  der  Paussiden  mit 
biologischen   und   phylogenetischen   Bemerkungen '    (Notes  from  the  Leyden 
Museum.,  XXV,  1904). 


the  Carabidae,  and  these  again  to  other  families  of  beetles,  the 
real  extent  of  the  natural  species  in  question  is  probably  much 
greater.  With  still  greater  certainty  may  all  the  varieties 
of  Staphylinidae  belonging  to  the  group  Lomechusa  be  regarded 
as  forming  a  natural  species.  We  may  therefore  rightly  say  : 
All  the  Lomechusini  form  one  natural  species  and  not  more 
than  one.  But  we  do  not  mean  to  limit  the  extent  of  this 
natural  species  to  the  Lomechusini)  for  this  group  of  Staphylinidae 
is  connected  phylogenetically  with  other  groups  of  the  same 
family,  and  the  whole  family  of  Staphylinidae  with  other  families 
of  beetles,  &c. 

If  we  consider  the  numerous  genera  and  species  of  ants 
from  the  earliest  Jurassic  period  to  the  present  day,  we  can 
hardly  doubt  that  they  are  offshoots  of  one  single  natural 
species,  and  are  not  several  natural  species.  The  same  remark 
applies  to  the  family  of  termites,  with  its  great  variety  of  fossil 
and  still  existent  genera  and  species.1  If  we  trace  back 
the  history  of  the  primitive  varieties  of  the  Palaeozoic  age, 
which  even  then  formed  several  distinct  classes,  whence  our 
present  orders  of  insects  branched  off  probably  in  the  Mesozoic 
age,2  we  may  succeed  perhaps,  in  course  of  time,  in  proving 
these  varieties  of  primitive  insects  to  be  offshoots  of  some 
original  stock,  which  possibly  is  connected  with  the  earliest 
marine  Arthropoda,  so  that  eventually  many  hundreds  of 
thousands  of  systematic  species  may  unite  to  form  one  single 
line,  one  single  natural  species. 

This  is  at  present  all  a  matter  of  pure  hypothesis  ;  but 
these  examples  serve  to  show  plainly  that  the  limits  to  be 
assigned  to  the  natural  species  become  more  and  more  uncertain 
the  higher  the  division  of  the  animal  system  and  the  more 
remote  the  historical  period  of  animal  life  under  consideration. 
It  will  therefore  be  best  for  practical  purposes  to  describe  as 
natural  species  only  those  groups  of  forms  which  investigation 
has  shown  with  sufficient  probability  to  be  uniform  genealogical 

Thus,  for  instance,  we  may  class  as  one  natural  species  all 
the  present  varieties  of  horse  (Equidae)  and  their  fossil  ancestors, 
comprising  various  systematic  genera,  although  we  do  not 

1  See  p.  276. 

2  Cf.  A.  Handlirsch,  Die  fossilen  Insekten,  Leipzig,  1906. 


yet  know  how  far  the  limits  of  this  natural  species  may  be 
extended  into  the  past  of  which  palseontology  takes  account.1 
Among  Molluscs,  the  Ammonites  may  be  mentioned  as  a 
group  of  forms  very  rich  in  systematic  families,  genera,  and 
species  ;  they  can  be  traced  from  the  Devonian  to  the  Cretaceous 
period  through  a  long  series  of  geological  strata,  as  a  uniform, 
close  line  of  forms,  that  we  must  reckon  as  all  belonging  to 
one  natural  species,  not  to  many.  I  might  add  many  other 
instances,  but  those  already  given  will  suffice  to  show  that 
the  distinction  between  systematic  and  natural  species  is  by 
no  means  devoid  of  actual  foundation.  It  is  in  fact  practically 
necessary,  if  we  are  to  have  a  scientific  knowledge  of  com- 
parative morphology  and  biology.3 

Secondly  :  The  distinction  is  of  far  greater  importance  from 
the  point  of  view  of  philosophy.  It  supplies  us  with  a  firm 
philosophical  basis,  upon  which  the  theories  of  creation  and 
descent  can  easily  be  reconciled  with  one  another.  It  is 
obvious  that  the  possession  of  such  a  basis  is  of  the  utmost 
importance  to  those  concerned  with  the  defence  of  Christi- 
anity. Our  monistic  opponents  are  fond  of  adopting  the 
device  of  directing  their  attacks  against  the  theory  of  per- 
manence, when  they  are  really  aiming  them  at  the  theory 
of  creation.  They  declare  the  two  theories  to  be  identical, 
and  hope,  by  overthrowing  the  one,  to  secure  the  downfall 
of  the  other.  But  their  hopes  are  doomed  to  disappoint- 
ment, if  we  resolutely  maintain  the  distinction  just  laid 
down.  If  we  believe  that  only  the  natural  species  in  their  primi- 
tive forms  were  created,  but  that  it  is  left  to  natural  science  to 
determine  the  number  and  extent  of  these  series  of  natural  forms, 
as  well  as  the  character  of  the  primitive  forms  themselves,  then 
the  enemies  of  the  Christian  cosmogony  will  no  longer  be  able  to 
taunt  us  with  having  to  accept  the  permanence  of  the  systematic 
species  as  an  article  of  faith.3  What  has  it  to  do  with  theistic 
cosmogony  whether  a  hare  and  a  rabbit,  a  horse  and  an  ass 
are  related  or  not  ?  The  recognition  of  a  personal  God,  the 

1  Fleischmann's  criticism  of  '  the  stock  instance  of  the  theory  of  descent ' 
(Die  Deszendenztheorie,  chapter  v)  seems  only  to  confirm  the  above  statement, 
and  not  to  prove  much  against  the  relationship  of  the  Equidae  to  one  another. 

2  Further  information  on  this  subject,  derived  from  my  own  investigations, 
will  be  found  in  the  next  chapter. 

3  This   italicised  passage  gives  the  reason  for  the  bitter  attacks  made  by 
monists  upon  the  '  natural  species.' 


Creator  of  all  finite  beings,  is  no  more  inseparably  connected 
with  the  theory  of  permanence  in  zoology  and  botany  than 
it  was  with  the  geocentric  system  in  astronomy. 

If  the  theory  of  descent  holds  its  ground,  and  takes  the  place  of 
the  old  theory  of  permanence,  the  theory  of  creation,  and  with 
it  the  Christian  cosmogony,  remains  as  firmly  established  as  ever. 
Indeed  the  Creator's  wisdom  and  power  are  revealed  in  a  more 
brilliant  light  than  ever,  as  this  theory  shows  the  organic  world 
to  have  assumed  its  present  form,  not  in  consequence  of  God's 
constant  interference  with  the  natural  order,  but  as  a  result 
of  the  action  of  those  laws  which  He  Himself  has  imposed  upon 

We  see  therefore  that,  in  this  department  also,  true  science 
leads  us  finally  to  a  fuller  recognition  of  God.1  It  is  a  mere 
delusion  on  the  part  of  modern  atheism,  in  its  various  forms 
and  shades  of  opinion,  to  fancy  that  the  theory  of  evolution 
has  enabled  the  world  to  dispense  with  a  Creator  ;  for,  the 
more  manifold  and  the  more  independent  is  the  evolution  of 
the  organic  world  according  to  the  laws  inherent  in  it,  the 
greater  must  be  the  wisdom  and  power  of  the  law-giver  who 
created  this  world.  The  Darwinian,  or  rather  Haeckelian, 
theory  of  chance,  which  derives  all  the  conformity  to  law  in 
nature  from  an  original  lawless  chaos,  by  means  simply  of  '  the 
survival  of  the  fittest,'  may  at  the  present  day  be  said  to  be 
discarded  by  science.  But  the  monistic  view  of  the  universe, 
which  professes  to  find  the  first  cause  of  the  orderly  arrangement 
of  the  world  in  the  world  itself,  and  not  in  a  personal  Creator 
substantially  distinct  from  it,  is  no  better  than  the  material- 
istic theory  of  chance  ;  for  the  so-called  God  of  monism,  whom 
it  identifies  with  tho  world  and  everything  therein,  proves  to 
be  a  true  medley  of  irreconcilable  and  inexplicable  contra- 
dictions, when  considered  in  the  light  of  sound  reason.  We  are 
told  that  God  is  the  most  perfect  being,  having  from  all  eternity 
the  ground  of  His  existence  in  Himself  ;  but  at  the  same  time 
He  is  a  God  who  must  develop  His  own  being  in  and  through 
the  world.  Such  a  monistic  God  would  be  pitiably  incomplete 
and  dependent,  for  His  very  existence  would  depend  upon  the 

1  On  this  subject  see  K.  Braun,  fiber  Kosmogonie  vom  Standpunkt  christlicher 
Wissenschaft,  1905,  especially  chapters  8  and  9.  Also  J.  Reinke,  '  Darf  die 
Natur  uns  als  Offenbarung  Gottes  gelten  ? '  (Turmer  Jahrbnch,  pp.  139-167, 
especially  pp.  162,  &c.). 


existence  of  every  midge,  and  fly,  and  creature  in  which 
He  develops  Himself.  To  have  invented  such  an  idea  of  God 
and  to  seek  to  make  it  take  the  place  of  the  theistic  conception 
of  Him,  are  achievements  of  modern  lack  of  thought,  not  o 
modern  science.  But,  on  the  contrary,  the  recognition  of  a 
personal  God,  who,  in  virtue  of  the  fulness  of  His  own  being, 
created  the  world  out  of  nothing,  is  still  demanded  by  sound 
human  understanding,  and  is  therefore  a  true  postulate  of 
science.1  Although  God  is  present  and  acts  in  all  His  creatures, 
He  is  essentially  distinct  from  the  world  and  independent  of 
it,  and  has  shone  forth  from  all  eternity  with  the  same  un- 
changing purity  and  perfection.  All  the  ephemeral  deities 
of  modern  monism  must  give  way  to  this  only  true  God  of 

At  the  present  day  men  are  fond  of  attacking  the  theistic 
cosmogony  by  saying  it  is  an  '  untenable  dualism '  to  recognise 
a  God  as  essentially  distinct  from  the  world.  Nobody  has 
yet  proved  this  dualism  to  be  untenable,  though  monism 
certainly  is  so.  I  am  not  one  of  those  who  '  prefer  the  most 
pitiable  confusion  to  dualism '  (C.  Stumpf).  There  is  in 
reality  onlyone  true  kind  of  monism,  and  that  is  the  unity. 
oMhelirst Tause  5f_all  rniit^Jbejng^^od  in  His  infinitjT2 
People  are  tond  of  quoting  (Jharles  Darwin  as  an  authority  in 
support  of  the  modern  theory  of  evolution,  but  he  did  not 
feel  that  blind  hatred  of  the  Creator  which  characterises 
Haeckelism.  Although  we  know  from  some  of  his  later  state- 
ments that  he  inclined  to  agnosticism,  he  never  altered  the 
closing  words  of  his  chief  work,  the  '  Origin  of  Species.'  Even 
in  the  sixth  edition,  published  in  1888,  after  his  death,  this 
beautiful  passage  occurs  :  '  There  is  grandeur  in  this  view 
of  life,  with  its  several  powers,  having  been  o